ALBERT

Description: The North Anatolian Fault Zone (NAFZ) starts to branch off in the western Bolu plain. The branches of the NAFZ in this location create the Almacık block which is surrounded by the latest surface ruptures of significant earthquakes that occurred between 1944 and 1999, but its northeastern part remains unruptured. The most recently formed rupture, that was a result of the 1999 November 12 Düzce earthquake, ended to the northwest of the Bakacak Fault. The connection between the Bakacak Fault and the main branch of the NAFZ via the Bolu plain has until now remained unknown. This paper establishes that the route of the missing link runs through the Dağkent, Kasaplar and Bürnük faults, a finding achieved with the help of seismic reflection studies. The paper also argues that the cross cutting nature of these newly determined faults and a stress analysis based on focal mechanism solutions of recent earthquakes demonstrate the termination of the suggested pull-apart nature of the Bolu plain.

Description: The source parameters and slip distribution of the 2013 May 11 M w 6.1 Minab earthquake are studied using seismology, geodesy and field observations. We observe left-lateral strike-slip motion on a fault striking ENE–WSW; approximately perpendicular to previously studied faults in the Minab–Zendan–Palami fault zone. The fault that ruptured in 2013 is one of a series of ~E–W striking left-lateral faults visible in the geology and geomorphology. These accommodate a velocity field equivalent to right-lateral shear on ~N–S striking planes by clockwise rotations about vertical axes. The presence of these faults can reconcile differences in estimates of fault slip rates in the western Makran from GPS and Quaternary dating. The longitudinal range of shear in the western Makran is likely to be controlled by the distance over which the underthrusting Arabian lithosphere deepens in the transition from continent–continent collision in the Zagros to oceanic subduction in the Makran.

Description: The Earth's outer core is a rotating ellipsoidal shell of compressible, stratified and self-gravitating fluid. As such, in the treatment of geophysical problems a realistic model of this body needs to be considered. In this work, we consider compressible and stratified fluid core models with different stratification parameters, related to the local Brunt-Väisälä frequency, in order to study the effects of the core's density stratification on the frequencies of some of the inertial-gravity modes of this body. The inertial-gravity modes of the core are free oscillations with periods longer than 12 hr. Historically, an incompressible and homogeneous fluid is considered to study these modes and analytical solutions are known for the frequencies and the displacement eigenfunctions of a spherical model. We show that for a compressible and stratified spherical core model the effects of non-neutral density stratification may be significant, and the frequencies of these modes may change from model to model. For example, for a spherical core model the frequency of the spin-over mode, the (2, 1, 1) mode, is unaffected while that of the (4, 1, 1) mode is changed from –0.410 for the Poincaré core model to –0.434, –0.447 and –0.483 for core models with the stability parameter β = –0.001, –0.002 and –0.005, respectively, a maximum change of about 18 per cent when β = –0.005. Our results also show that for small stratification parameter, |β| ≤ 0.005, the frequency of an inertial-gravity mode is a nearly linear function of β but the slope of the line is different for different modes, and that the effects of density stratification on the frequency of a mode is likely related to its spatial structure, which remains the same in different Earth models. We also compute the frequencies of some of the modes of the ‘PREM’ (spherical shell) core model and show that the frequencies of these modes may also be significantly affected by non-zero β.

Description: Reaching sub-salt hydrocarbon targets in the deeper part of the Gulf of Mexico requires drilling through a salt canopy. The suture zones in the salt canopy are potential drilling hazards due to anomalous pressure behaviour of entrapped sediments. The Pólya vector field of coalescing salt sheets inside the canopy is used to explain suture formation and distinguish between upright and inclined suture contacts. Our analytical models, based on complex potentials, provide exact solutions for multiple source flows as they compete for space when spreading into the viscous continuum of the salt canopy. The velocity gradient tensor yields the strain rate tensor, which is used to map the principal strain rate magnitude inside the canopy. Quantification of one of the principal strain rates is sufficient because the plane deformation assumption ensures the two principal strain rates are equal in magnitude (but of opposite sign); the third principal dimension can have neither strain nor deviatoric stress. Visualization of the locations where the principal stress vanishes or peaks (with highs and lows) is useful for pre-drilling plans because such peaks must be avoided and the stress-free locations provide the safer drilling sites. A case study—of the Walker Ridge region—demonstrates the practical application of our new method.

Description: The Indochina block is important to our understanding of the extrusion model as a consequence of the Indo-Eurasia collision. The lithospheric structure of this block, however, remains obscured due to a lack of sufficient instrumentation for high resolution seismic imaging. We present a shear velocity model derived from Rayleigh wave phase velocity tomography using data from recently deployed seismic networks in this region. Our inversion results for lithospheric structure show strong correlations with tectonic history in this block. A prominent slow-velocity anomaly (5 per cent) is observed in northern Indochina along the Ailao Shan-Red River (ASRR) shear zone including Chuxiong basin, Lanping-Simao fold belt and Thailand rift basin, which has seen extensive deformation events since Eocene. The Khorat Plateau basin is characterized by thick continental keel type lithosphere, consistent with palaeomagnetic and geological observations indicating this basin has experienced much less deformation than the surrounding regions. Additionally, our inversion imaged a sharp, lithospheric-scale velocity contrast across the southeastern segment of ASRR, indicative of a thin and thus relatively weak lithosphere southwest of Red River Fault. The thin lithosphere, low asthenospheric seismic velocities we observe and the average crustal thicknesses in the region suggest that the topography high is dynamically supported by upwelling asthenosphere rather than thickening of the crust/lithosphere. Based on the occurrence of Palaeogene volcanism and its timing, we prefer an explanation of thinning of the lithosphere and allowing a throughgoing fault rather than emplacement of a thin terrane to explain the thin lithosphere. Therefore, the anomalously thin lithosphere between Khorat Plateau and the ASRR in conjunction with other geological observations is generally consistent with the extrusion model for Indochina, which requires localization of lithospheric deformation around tectonic blocks.

Description: On Earth, oceanic plates subduct beneath a variety of overriding plate (OP) styles, from relatively thin and negatively buoyant oceanic OPs to thick and neutrally/positively buoyant continental OPs. The inclusion of an OP in numerical models of self-consistent subduction has been shown to reduce the rate that subducting slabs roll back relative to the equivalent single plate models. We use dynamic, 2-D subduction models to investigate how the mechanical properties, namely viscosity, thickness, and density, of the OP modify the slab rollback rate and the state of stress of the OP. In addition, we examine the role of the subducting plate (SP) viscosity. Because OP deformation accommodates the difference between the slab rollback rate and the far-field OP velocity, we find that the temporal variations in the rollback rate results in temporal variations in OP stress. The slabs in our models roll back rapidly until they reach the lower mantle viscosity increase, at which point the rollback velocity decreases. Concurrent with this reduction in rollback rate is a switch from an OP dominated by extensional stresses to a compressional OP. As in single plate models, the viscosity of the SP exerts a strong control on subducting slab kinematics; weaker slabs exhibit elevated sinking velocities and rollback rates. The SP viscosity also exerts a strong control on the OP stress regime. Weak slabs, either due to reduced bulk viscosity or stress-dependent weakening, have compressional OPs, while strong slabs have dominantly extensional OPs. While varying the viscosity of the OP alone does not substantially affect the OP stress state, we find that the OP thickness and buoyancy plays a substantial role in dictating the rate of slab rollback and OP stress state. Models with thick and/or negatively buoyant OPs have reduced rollback rates, and increased slab dip angles, relative to slabs with thin and/or positively buoyant OPs. Such elevated trench rollback for models with positively buoyant OPs induces extensional stresses in the OP, while OPs that are strongly negatively buoyant are under compression. While rollback is driven by the negative buoyancy of the subducting slab in such models of free subduction, we conclude that the physical properties of the OP potentially play a significant role in modulating both rollback rates and OP deformation style on Earth.

Description: In the central Andes, the Nazca plate displays large along strike variations in dip with a near horizontal subduction angle between 28 and 32°S referred to the Pampean flat slab segment. The upper plate above the Pampean flat slab has high rates of crustal seismicity and active basement cored uplifts. The SIEMBRA experiment, a 43-broad-band-seismic-station array was deployed to better characterize the Pampean flat slab region around 31°S. In this study, we explore the lithospheric structure above the flat slab as a whole and its relation to seismicity. We use the SIEMBRA data to perform a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion to constrain the shear wave velocity variations in the lithosphere. Our joint inversion results show: (1) the presence of several upper-plate mid-crustal discontinuities and their lateral extent that are probably related to the terrane accretion history; (2) zones of high shear wave velocity in the upper-plate lower crust associated with a weak Moho signal consistent with the hypothesis of partial eclogitization in the lower crust; (3) the presence of low shear-wave velocities at ~100 km depth interpreted as the subducting oceanic crust. Finally, in order to investigate the relation of the lithospheric structure to seismicity, we determine an optimal velocity–depth model based on the joint inversion results and use it to perform regional moment tensor inversions (SMTI) of crustal and slab earthquakes. The SMTI for 18 earthquakes that occurred between 2007 and 2009 in the flat slab region below Argentina, indicates systematically shallower focal depths for slab earthquakes (compared with inversions using previous velocity models). This suggests that the slab seismicity is concentrated mostly between 90 and 110 km depths within the subducting Nazca plate's oceanic crust and likely related to dehydration. In addition, the slab earthquakes exhibit extensional focal mechanisms suggesting new faulting at the edges of the flat portion of the slab. SMTI solutions for upper-plate crustal earthquakes match well the geological observations of reactivated structures and agree with crustal shortening. Our new constraints on flat slab structure can aid earthquake characterization for regional seismic hazard assessment and efforts to help understand the mechanisms for slab flattening in the central Andes.

Description: The elastic displacement and stress fields due to rectangular faults and opening-mode fractures within an anisotropic homogeneous half-space are derived in this paper. The solution is expressed in terms of the mathematically elegant and computationally powerful Stroh formalism and can be applied to the generally anisotropic half-space or a transversely isotropic half-space with any oriented isotropic plane. For any flat fault or opening-mode fracture of polygonal shape, one needs only to carry out a simple line integral from 0 to in order to express the fault-induced response. Numerical examples are presented to demonstrate the effect of the anisotropy and fault orientation on the internal and surface responses of the half-space. Our results prove that both rock anisotropy and fault orientation could dramatically change the fields in the domain and one needs to consider these properties as accurately as possible to be able to predict the response in the domain precisely. Anisotropy of the rock mass may alter the dominant displacement and stress components at observation points in the model domain as compared to the isotropic case.

Description: Shear deformation of partially molten rock in laboratory experiments causes the emergence of melt-enriched sheets (bands in cross-section) that are aligned at about 15°–20° to the shear plane. Deformation and deviatoric stress also cause the coherent alignment of pores at the grain scale. This leads to a melt-preferred orientation that may, in turn, give rise to an anisotropic permeability. Here we develop a simple, general model of anisotropic permeability in partially molten rocks. We use linearized analysis and nonlinear numerical solutions to investigate its behaviour under simple-shear deformation. In particular, we consider implications of the model for the emergence and angle of melt-rich bands. Anisotropic permeability affects the angle of bands and, in a certain parameter regime, it can give rise to low angles consistent with experiments. However, the conditions required for this regime have a narrow range and seem unlikely to be entirely met by experiments. Anisotropic permeability may nonetheless affect melt transport and the behaviour of partially molten rocks in Earth's mantle.

Description: Eastern Anatolia region between north–south colliding Arabian and Eurasian plates has no significant crustal root and shallow (upper) mantle flow beneath seems to be vertically supporting its high topography. It has a high surface heat flow and the underlying mantle is characterized by low seismic velocity zones. Using a mantle density/temperature variation field derived from P -wave seismic velocity, current shallow mantle flow and resultant dynamic topography of Eastern Anatolia and adjacent Arabian foreland and Caucasus areas were calculated along a vertical section. The section crosses the tectonic boundaries interrelated with slab bodies (high seismic velocity/cold regions) and the low velocity zones above the slabs. According to the modelling experiments, the surface topography of Eastern Anatolia seems to be supported by shallow mantle flow dynamics. On the other hand, residual topography for the region was calculated using high resolution crustal thickness data. Positive residual topography that suggests an undercompensated state of Eastern Anatolia is in concordance with the dynamic topography anomaly. The modelled local shallow mantle flow support due to the density contrast between hot (low velocity) zones and underlying cold slab bodies beneath the area may be the present-day snapshot of the mantle flow uplift in Eastern Anatolia presence of which was previously suggested.

Description: Observations of the temporal variations in the volume flux of a plume can provide useful constraints on geodynamic models of plumes and plume-plate interactions. Furthermore, they can be compared with observations at other plumes and may be analysed further to understand the nature and cause of the variations. The volume plume flux is typically derived from a sum of edifice and compensation root volumes. The former can be obtained via the application of regional–residual separation procedures that split the observed relief into regional (swell) and residual (edifice) components, while the latter is generally inferred from the former using local (Airy) or regional (flexural) compensation models. Most regional–residual techniques used in past studies give non-unique results and provide no estimates of the uncertainty in the separation, which impacts the significance of the results. Here, the optimal robust separator (ORS) method achieves a unique separation for the swell and edifice components of the Hawaiian Ridge and furthermore obtain confidence bounds on the total volume flux. A fast spectral method for plate flexure with different edifice and infill densities is used to determine compensation volumes. Although my flux estimates have assigned confidence bounds, these are much smaller than the flux estimates themselves. A comparison of my new results to published volume flux curves shows that my revised flux estimates are lower by a factor of 2–3. Reproducing the prior higher results demonstrates that these discrepancies appear to be related to shortcomings in the implementation of the methodology used in the separation. The variability in the Hawaiian plume flux occurs at two different time scales: A short (1–2 Myr) periodicity related to the spacing of islands and seamounts, which ultimately is related to plume-plate flexural interactions, and a much longer (10–15 Myr) periodicity that may be related to plate kinematic changes. Superimposed on these trends may be an exponential increase towards more recent times, but this trend may also be explained by a higher flux level during the period when the plume was positioned beneath the relatively younger lithospheric segment bracketed by the Murray and Molokai fracture zones. Landslides and erosion of the edifice may imply an underestimation of total volumes by 5–10 per cent. The main uncertainty facing studies of plume flux is related to the unknown quantity of magmatic underplating.

Description: The Castrovillari scarps (Cfs) are located in northern Calabria (Italy) and consist of three main WSW-dipping fault scarps resulting from multiple rupture events. At the surface, these scarps are defined by multiple breaks in slope. Despite its near-surface complexity, the faults likely merge to form a single normal fault at about 200 m depth, which we refer to as the Castrovillari fault. We present the results of a multidisciplinary and multiscale study at a selected site of the Cfs with the aim to (i) characterize the geometry at the surface and at depth and (ii) obtain constraints on the fault slip history. We investigate the site by merging data from quantitative geomorphological analyses, electrical resistivity and ground penetrating radar surveys, and palaeoseismological trenching along a ~40 m high scarp. The closely spaced investigations allow us to reconstruct the shallow stratigraphy, define the fault locations, and measure the faulted stratigraphic offsets down to 20 m depth. Despite the varying resolutions, each of the adopted approaches suggests the presence of sub-parallel fault planes below the scarps at approximately the same location. The merged datasets permit the evaluation of the fault array (along strike for 220 m within a 370-m-wide zone). The main fault zone consists of two closely spaced NW–SE striking fault planes in the upper portion of the scarp slope and another fault at the scarp foot. The 3-D image of the fault surfaces shows west to southwest dipping planes with values between 70° and 80°; the two closely spaced planes join at about 200 m below the surface. The 8-to-12-m-high upper fault, which shows the higher vertical displacements, accommodated most of the deformation during the Holocene. Results from the trenching analysis indicate a minimum slip per event of 0.6 m and a maximum short-term slip rate of 0.6 mm yr –1 for the Cf. The shallow subsurface imaging techniques are particularly helpful in evaluating the possible field uncertainties related to postfaulting modification by erosional/depositional/human processes, such as within stream valleys and urbanized zones.

Description: With the progress of mantle convection modelling over the last decade, it now becomes possible to solve for the dynamics of the interior flow and the surface tectonics to first order. We show here that tectonic data (like surface kinematics and seafloor age distribution) and mantle convection models with plate-like behaviour can in principle be combined to reconstruct mantle convection. We present a sequential data assimilation method, based on suboptimal schemes derived from the Kalman filter, where surface velocities and seafloor age maps are not used as boundary conditions for the flow, but as data to assimilate. Two stages (a forecast followed by an analysis) are repeated sequentially to take into account data observed at different times. Whenever observations are available, an analysis infers the most probable state of the mantle at this time, considering a prior guess (supplied by the forecast) and the new observations at hand, using the classical best linear unbiased estimate. Between two observation times, the evolution of the mantle is governed by the forward model of mantle convection. This method is applied to synthetic 2-D spherical annulus mantle cases to evaluate its efficiency. We compare the reference evolutions to the estimations obtained by data assimilation. Two parameters control the behaviour of the scheme: the time between two analyses, and the amplitude of noise in the synthetic observations. Our technique proves to be efficient in retrieving temperature field evolutions provided the time between two analyses is 10 Myr. If the amplitude of the a priori error on the observations is large (30 per cent), our method provides a better estimate of surface tectonics than the observations, taking advantage of the information within the physics of convection.

Description: In this study, the Moho depth is estimated based on the refined spherical Bouguer gravity disturbance and DTM2006 topographic data using the Vening Meinesz-Moritz gravimetric-isostatic hypothesis. In this context, we compute the refined spherical Bouguer gravity disturbances in a set of 1° x 1° blocks. The spherical terrain correction, a residual correction to each Bouguer shell, is computed using rock heights and ice sheet thicknesses from the DTM2006 and Earth2014 models. The study illustrates that the defined simple Bouguer gravity disturbance corrected for the density variations of the oceans, ice sheets and sediment basins and also the non-isostatic effects needs a significant terrain correction to become the refined Bouguer gravity disturbance, and that the isostatic gravity disturbance is significantly better defined by the latter disturbance plus a compensation attraction. Our study shows that despite the fact that the lateral variation of the crustal depth is rather smooth, the terrain affects the result most significantly in many areas. The global numerical results show that the estimated Moho depths by the simple and refined spherical Bouguer gravity disturbances and the seismic CRUST1.0 model agree to 5.6 and 2.7 km in RMS, respectively. Also, the mean value differences are 1.7 and 0.2 km, respectively. Two regional numerical studies show that the RMS differences between the Moho depths estimated based on the simple and refined spherical Bouguer gravity disturbance and that using CRUST1.0 model yield fits of 4.9 and 3.2 km in South America and yield 3.2 and 3.4 km in Fennoscandia, respectively.

Description: The rifting episode that occurred in Dabbahu–Manda–Hararo (Ethiopia) between 2005 and 2010 during which 14 dyke intrusions were emitted, was a unique opportunity to study interactions between tectonic deformation and magmatic processes. While magmatism has been shown to control primarily the spatial and temporal distribution of dyke intrusions during this accretion sequence, the role of faults in accommodating plate spreading in rift segments is poorly understood. During interdyking periods, transient ground deformation due to magma movement is generally observed. Investigating such a small-scale deformation and in particular the movement along faults during these periods will help understanding the factors that trigger fault movement in magmatic rifts. We analyse fault activity during three interdyking periods: 2006 December–June (d0–d1), 2007 January–July (d5–d6) and 2009 November–January (d10–d11). The time–space evolution of surface displacements along ~700 faults is derived from pairs of ascending and descending SAR interferograms. Surface slip distributions are then compared with codyking ground deformation fields. The results show that faults are mainly activated above the areas affected by magma emplacement during interdyking periods. A detailed analysis of brittle deformation during the six months following the 2005 September intrusion shows asymmetric deformation on the rift shoulders, with significant opening on faults located to the west of the dyke. We explain this feature by the activation of westward dipping pre-existing faults, with block rotations in between. In addition, we observe that the strip encompassing the activated faults narrows by 30 per cent from co- to interdyking period. This suggests that magma keeps migrating to shallower depths after the dyke intrusion. During a rifting episode, activation of faults in a pre-existing fracture network therefore seems to be mainly controlled by deep magma processes.

Description: We investigate how uncertainties in seismic and density structure of the crust propagate to uncertainties in mantle density structure. The analysis is based on interpretation of residual upper-mantle gravity anomalies which are calculated by subtracting (stripping) the gravitational effect of the crust from the observed satellite gravity field data (GOCE Direct release 3). Thus calculated residual mantle gravity anomalies are caused mainly by a heterogeneous density distribution in the upper mantle. Given a relatively small range of expected compositional density variations in the lithospheric mantle, knowledge on uncertainties associated with incomplete information on crustal structure is of utmost importance for progress in gravity modelling. Uncertainties in the residual upper-mantle gravity anomalies result chiefly from uncertainties in (i) seismic V P velocity–density conversion for the crust and (ii) uncertainties in the seismic crustal structure (thickness and average V P velocities of individual crustal layers, including the sedimentary cover). We examine the propagation of these uncertainties into determinations of lithospheric mantle density and analyse both sources of possible uncertainties by applying different velocity-to-density conversions and by introducing variations into the crustal structure which correspond to typical resolution of high-quality and low-quality seismic models. We apply our analysis to Siberia (the West Siberian Basin and the Siberian Craton) for which a new regional seismic crustal model, SibCrust, has recently become available. For the same region, we also compute upper-mantle gravity and density anomalies based on three global crustal models (CRUST 5.1, CRUST 2.0 and CRUST 1.0) and compare the results based on four different crustal models. A large uncertainty in the V P -to-density conversion may result in the uncertainty in lithospheric mantle density anomalies of ca. 0.02–0.03 g cm –3 (i.e. 0.5–1 per cent, which is comparable to compositional density anomalies expected for continental lithosphere mantle). Similar values of uncertainties may be caused by a 0.2 km s –1 error in average crustal V P velocities or by a 2 km uncertainty in the Moho depth. One of the largest uncertainties is caused by errors in thickness of the sedimentary layer, and a 2 km error leads to ca. 0.03 g cm –3 error in lithospheric mantle densities. Large deviations (locally ±10 km) of the Moho depth in global crustal models (CRUST 5.1, CRUST2.0 and CRUST1.0) from the high-resolution regional seismic model of the crust, SibCrust, may produce artefact residual mantle gravity anomalies of up to ±150 mGal locally, caused by large errors in crustal gravity corrections. These errors in gravity anomalies produce up to ca. 0.04 g cm –3 ( ca. 1.2 per cent) errors in density of the lithospheric mantle, which may well correspond to the amplitude of real density anomalies in the mantle. Our results demonstrate that gravity modelling alone cannot reliably constrain the crustal structure, including the Moho depth and thickness of sediments.

Description: The Pyrenean mountain range is a slowly deforming belt with continuous and moderate seismic activity. To quantify its deformation field, we present the velocity field estimated from a GPS survey of the Pyrenees spanning 18 yr. The PotSis and ResPyr networks, including a total of 85 GPS sites, were installed and first measured in 1992 and 1995–1997, respectively, and remeasured in 2008 and 2010. We obtain a deformation field with velocities less than 1 mm yr –1 across the range. The estimated velocities for individual stations do not differ significantly from zero with 95 per cent confidence. Even so, we estimate a maximum extensional horizontal strain rate of 2.0 ± 1.7 nanostrain per year in a N–S direction in the western part of the range. We do not interpret the vertical displacements due to their large uncertainties. In order to compare the horizontal strain rates with the seismic activity, we analyse a set of 194 focal mechanisms using three methods: (i) the ‘r’ factor relating their P and T axes, (ii) the stress tensors obtained by fault slip inversion and (iii) the strain-rate tensors. Stress and strain-rate tensors are estimated for: (i) the whole data set, (ii) the eastern and western parts of the range separately, and (iii) eight zones, which are defined based on the seismicity and the tectonic patterns of the Pyrenees. Each of these analyses reveals a lateral variation of the deformation style from compression and extension in the east to extension and strike-slip in the west of the range. Although the horizontal components of the strain-rate tensors estimated from the seismic data are slightly smaller in magnitude than those computed from the GPS velocity field, they are consistent within the 2 uncertainties. Furthermore, the orientations of their principal axes agree with the mapped active faults.

Description: The resistivity structure of the lithospheric mantle beneath the Proterozoic Grenville Province in southern Ontario, Canada is investigated using 84 magnetotelluric (MT) sites divided into four profiles. Depth-based regional geoelectric dimensionality analyses of the MT responses indicate that the mantle lithosphere north of Lake Ontario can be subdivided into upper (45–150 km) and deeper (〉200 km) lithospheric mantle layers with regional strike azimuths of N85°E (±5°) and N65°E (±5°), respectively. MT responses from the Grenville Front and the northwest part of the Central Gneiss Belt are compatible with the presence of 2-D resistivity structures but farther to the southeast, in the southeast part of the Central Gneiss Belt and Central Metasedimentary Belt, they suggest the presence of localized 3-D structures. 2-D inversion of distortion-free MT responses images a large scale very resistive (〉20 000  m) region that extends 300 km southeast of the Grenville Front and for at least 800 km along-strike in the lithospheric mantle beneath the Grenville Province. This feature is interpreted to be Superior Province lithosphere and the corresponding N85°E geoelectric strike to be associated with the fabric of the Superior Province. The base of the resistor reaches depths of 280 km on two of the three MT profiles north of Lake Ontario and this depth is interpreted to be the base of the lithosphere. A large region of enhanced conductivity in the lower lithosphere, spatially correlated with decreased seismic velocity, is bounded to the northwest by a subvertical resistivity anomaly located near the Kirkland Lake and Cobalt kimberlite fields. The enhanced conductivity in the lower lithosphere is attributed to refertilization by fluids associated with Cretaceous kimberlite magmatism and can be explained by water content in olivine of 50 wt ppm in background areas with higher values in a localized anomaly beneath the kimberlite fields. Farther to the southeast the resistivity models include a lithospheric conductor between 100 and 150 km depth beneath the Central Metasedimentary Belt. The enhanced conductivity is attributed to grain boundary graphite films, associated with the Cretaceous kimberlitic magmatic process, or to water and carbon, introduced into the mantle during the pre-Grenvillian tectonism.

Description: Displacements and stress-field changes associated with earthquakes, volcanoes, landslides and human activity are often simulated using numerical models in an attempt to understand the underlying processes and their governing physics. The application of elastic dislocation theory to these problems, however, may be biased because of numerical instabilities in the calculations. Here, we present a new method that is free of artefact singularities and numerical instabilities in analytical solutions for triangular dislocations (TDs) in both full-space and half-space. We apply the method to both the displacement and the stress fields. The entire 3-D Euclidean space $\mathbb {R}^{3}$ is divided into two complementary subspaces, in the sense that in each one, a particular analytical formulation fulfils the requirements for the ideal, artefact-free solution for a TD. The primary advantage of the presented method is that the development of our solutions involves neither numerical approximations nor series expansion methods. As a result, the final outputs are independent of the scale of the input parameters, including the size and position of the dislocation as well as its corresponding slip vector components. Our solutions are therefore well suited for application at various scales in geoscience, physics and engineering. We validate the solutions through comparison to other well-known analytical methods and provide the MATLAB codes.

Description: Models of the glacial isostatic adjustment process, which is dominated by the influence of the Late Pleistocene cycle of glaciation and deglaciation, depend on two fundamental inputs: a history of ice-sheet loading and a model of the radial variation of mantle viscosity. These models may be tested and refined by comparing their local predictions of relative sea level history to geological inferences based upon appropriate sea level indicators. The U.S. Atlantic coast is a region of particular interest in this regard, due to the fact that data from the length of this coast provides a transect of the forebulge associated with the former Laurentide ice sheet. High-quality relative sea level histories from this region are employed herein to explore the ability of current models of mantle viscosity to explain the inferred evolution of relative sea level that have accompanied forebulge collapse following deglaciation. Existing misfits are characterized, and alternatives are explored for their reconciliation. It is demonstrated that a new model of mantle viscosity, referred to herein as VM6, when coupled with the latest model of deglaciation history ICE-6G_C, is able to eliminate the majority of these misfits, while continuing to reconcile a wide range of other important geophysical observables, as well as additional relative sea level data from the North American. West coast which also record the collapse of the forebulge but which have not been employed in tuning the viscosity profile to enable ICE-6G_C (VM6) to fit the East coast data set.

Description: The origin of shear wave splitting (SWS) in the mantle beneath Iceland is examined using numerical models that simulate 3-D mantle flow and the development of seismic anisotropy due to lattice-preferred orientation (LPO). Using the simulated anisotropy structure, we compute synthetic SKS waveforms, invert them for fast polarization directions and split times, and then compare the predictions with the results from three observational studies of Iceland. Models that simulate a mantle plume interacting with the Mid-Atlantic Ridge in which the shallow-most mantle has a high viscosity due to the extraction of water with partial melting, or in which C-type olivine LPO fabric is present due to high water content in the plume, produce the largest chi-squared misfits to the SWS observations and are thus rejected. Models of a low-viscosity mantle plume with A-type olivine fabric everywhere, or with the added effects of E-type fabric in the plume below the solidus produce lower misfits. The lowest misfits are produced by models that include a rapid (~50 km Myr –1 ) northward regional flow (NRF) in the mid-upper mantle, either with or without a plume. NRF was previously indicated by a receiver function study and a regional tomography study, and is shown here to be a major cause of the azimuthal anisotropy beneath Iceland. The smallest misfits for the models with both a plume and NRF are produced when LPO forms above depths of 300–400 km, which, by implication, also mark the depths above which dislocation creep dominates over diffusion creep. This depth of transition between dislocation and diffusion creep is greater than expected beneath normal oceanic seafloor, and is attributed to the unusually rapid strain rates associated with an Iceland plume and the NRF.

Description: The resistivity structure of the lithospheric mantle beneath the Proterozoic Grenville Province in southern Ontario, Canada is investigated using 84 magnetotelluric (MT) sites divided into four profiles. Depth-based regional geoelectric dimensionality analyses of the MT responses indicate that the mantle lithosphere north of Lake Ontario can be subdivided into upper (45–150 km) and deeper (〉200 km) lithospheric mantle layers with regional strike azimuths of N85°E (±5°) and N65°E (±5°), respectively. MT responses from the Grenville Front and the northwest part of the Central Gneiss Belt are compatible with the presence of 2-D resistivity structures but farther to the southeast, in the southeast part of the Central Gneiss Belt and Central Metasedimentary Belt, they suggest the presence of localized 3-D structures. 2-D inversion of distortion-free MT responses images a large scale very resistive (〉20 000  m) region that extends 300 km southeast of the Grenville Front and for at least 800 km along-strike in the lithospheric mantle beneath the Grenville Province. This feature is interpreted to be Superior Province lithosphere and the corresponding N85°E geoelectric strike to be associated with the fabric of the Superior Province. The base of the resistor reaches depths of 280 km on two of the three MT profiles north of Lake Ontario and this depth is interpreted to be the base of the lithosphere. A large region of enhanced conductivity in the lower lithosphere, spatially correlated with decreased seismic velocity, is bounded to the northwest by a subvertical resistivity anomaly located near the Kirkland Lake and Cobalt kimberlite fields. The enhanced conductivity in the lower lithosphere is attributed to refertilization by fluids associated with Cretaceous kimberlite magmatism and can be explained by water content in olivine of 50 wt ppm in background areas with higher values in a localized anomaly beneath the kimberlite fields. Farther to the southeast the resistivity models include a lithospheric conductor between 100 and 150 km depth beneath the Central Metasedimentary Belt. The enhanced conductivity is attributed to grain boundary graphite films, associated with the Cretaceous kimberlitic magmatic process, or to water and carbon, introduced into the mantle during the pre-Grenvillian tectonism.

Description: Using dynamic rupture models of a right-lateral fault embedded in an elastic or elastoplastic 3-D medium, we investigate elastic and inelastic responses of compliant fault zones to nearby earthquake ruptures. We particularly examine effects of fault zone depth, width, shape and rigidity reduction on the surface displacement field. Our results from elastic models show that deeper and wider fault zones generally result in larger residual displacements. However, for shallow fault zones, the vertical residual displacement is insensitive to or even decreases with fault zone width. The width of horizontal displacement anomalies across a fault zone is only indicative of the fault zone width near the Earth's surface. There are trade-off effects among fault zone depth, width, shape and rigidity reduction on the amplitude of surface residual displacements. Our elastoplastic models show that plastic strain can occur along the entire fault zone near the Earth's surface and in the extensional quadrant at depth, if fault zone rocks are close to failure before a nearby earthquake happens. Compared with results from elastic models, plastic strain near the Earth's surface generally enhances surface displacements of a fault zone and does not change the trend of effects of fault zone depth and width, while plastic strain at depth can result in reduced retrograde motion or sympathetic motion across the fault zone, and introduce complexities in effects of fault zone depth and width. Sympathetic horizontal motion more likely occurs across a narrow fault zone with inelastic response at depth. Vertical motion in the extensional quadrant may actually decrease with fault zone width in elastoplastic models. Sympathetic horizontal motion, or small retrograde horizontal motion in conjunction with large vertical motion above a fault zone is indicative of inelastic response of a fault zone at depth.

Description: The Algerian margin formed through back-arc opening of the Algerian basin (Mediterranean Sea) resulting from the roll-back of the Tethyan slab. Recent geophysical data acquired along the Algerian margin showed evidence of active or recent compressive deformation in the basin due to the ongoing Africa–Eurasia convergence. Published data from four wide-angle seismic profiles have allowed imaging the deep structure of the Algerian margin and its adjacent basins. In this study, we converted these velocity models into density models, then into isostatic anomalies. This allowed us to image an isostatic disequilibrium (relative to a local isostasy model) reaching a maximum amplitude at the margin toe. Converting isostatic anomalies into Moho depth variations shows that the Moho extracted from wide-angle seismic data is deeper than the one predicted by a local isostasy model in the oceanic domain, and shallower than it in the continental domain. These anomalies can be interpreted by opposite flexures of two plates separated by a plate boundary located close to the margin toe. We use a finite element model to simulate the lithospheric flexure. The amplitude of the equivalent vertical Moho deflection is larger in the central part of the study area (6–7 km) than on the easternmost and westernmost profiles (3 km). The effective elastic thickness used to best match the computed deflection is always extremely low (always less than 10 km) and probably reflects the relatively low strength of the lithosphere close to the plate boundary. Comparison with other wide-angle seismic profiles across an active and a passive margin show that the North Algerian margin displays isostatic anomalies close to that of an active margin. Finally, plate flexure is highest at the southern tip of the ocean-continent transition, possibly indicating that a former passive margin detachment is reactivated as a crustal scale reverse fault pre-dating a future subduction.

Description: Between 2011 and 2013, two dense transects were deployed across the central and western Pyrenees to get better constraints on the deep lithospheric architecture and discriminate the competing models of the structure and formation of the Pyrenees. Each transect recorded the regional and global seismicity during a period of approximately 1 yr. Here, we exploit the records of teleseismic compressional waves and of their conversions to shear waves on internal discontinuities in order to map lithospheric interfaces beneath the two transects. The migrated sections, obtained by performing common conversion point stacks, are in remarkable agreement with the results of the ECORS-Pyrenees and ECORS-Arzacq deep seismic surveys. However, the migrations of converted waves reveal new details of the deep lithospheric architecture that could not be seen with the active source experiments. The new images provide clear and definite evidence for the subduction of a thinned Iberian crust down to at least ~70 km depth, a result that has important implications for the formation of the Pyrenees. The subduction of the Iberian lithosphere leads to reconsider the amount of convergence between Iberia and Europe during the Cenozoic. A recent regional P -wave tomography, relying on the data of the PYROPE and IBERARRAY temporary experiments, revealed the segmentation of lithospheric structures by inherited Hercynian NE–SW transfer faults that were reactivated during the Albian rifting. Our migration images are consistent with this model, and give further support to the idea that the Pyrenees were produced by the tectonic inversion of a segmented hyperextended rift that was buried by subduction beneath the European Plate.

Description: Post-seismic deformation is commonly attributed to viscoelastic relaxation and/or afterslip, although discerning between the two driving mechanisms can be difficult. A major complication in modeling post-seismic deformation is that forward models can be computationally expensive, making it difficult to adequately search model space to find the optimal fault slip distribution and lithospheric viscosity structure that can explain observable post-seismic deformation. We propose an inverse method which uses coseismic and early post-seismic deformation to rapidly and simultaneously estimate a fault slip history and an arbitrarily discretized viscosity structure of the lithosphere. Our method is based on an approximation which is applicable to the early post-seismic period and expresses surface deformation resulting from viscoelastic relaxation as a linearized function with respect to lithospheric fluidity. We demonstrate this approximation using two-dimensional earthquake models. We validate the approximation and our inverse method using two three-dimensional synthetic tests. The success of our synthetic tests suggests that our method is capable of distinguishing the mechanisms driving early post-seismic deformation and recovering an effective viscosity structure of the lithosphere.

Description: Geophysical data from the MEDOC experiment across the Northern Tyrrhenian backarc basin has mapped a failed rift during backarc extension of cratonic Variscan lithosphere. In contrast, data across the Central Tyrrhenian have revealed the presence of magmatic accretion followed by mantle exhumation after continental breakup. Here we analyse the MEDOC transect E–F, which extends from Sardinia to the Campania margin at 40.5°N, to define the distribution of geological domains in the transition from the complex Central Tyrrhenian to the extended continental crust of the Northern Tyrrhenian. The crust and uppermost mantle structure along this ~400-km-long transect have been investigated based on wide-angle seismic data, gravity modelling and multichannel seismic reflection imaging. The P -wave tomographic model together with a P -wave-velocity-derived density model and the multichannel seismic images reveal seven different domains along this transect, in contrast to the simpler structure to the south and north. The stretched continental crust under Sardinia margin abuts the magmatic crust of Cornaglia Terrace, where accretion likely occurred during backarc extension. Eastwards, around Secchi seamount, a second segment of thinned continental crust (7–8 km) is observed. Two short segments of magmatically modified continental crust are separated by the ~5-km-wide segment of the Vavilov basin possibly made of exhumed mantle rocks. The eastern segment of the 40.5°N transect E–F is characterized by continental crust extending from mainland Italy towards the Campania margin. Ground truthing and prior geophysical information obtained north and south of transect E–F was integrated in this study to map the spatial distribution of basement domains in the Central Tyrrhenian basin. The northward transition of crustal domains depicts a complex 3-D structure represented by abrupt spatial changes of magmatic and non-magmatic crustal domains. These observations imply rapid variations of magmatic activity difficult to reconcile with current models of extension of continental lithosphere essentially 2-D over long distances.

Description: Northern Attica in Greece is characterized by a set of north dipping, subparallel normal faults. These faults were considered to have low tectonic activity, based on historical earthquake reports, instrumental seismicity and slip rate estimates. This study presents new data for one of these faults, the Milesi Fault. We run GIS based geomorphological analyses on fault offset distribution, field mapping of postglacial fault scarps and ground penetrating radar profiling to image hangingwall deformation. The first palaeoseismological trenching in this part of Greece allowed obtaining direct data on slip rates and palaeoearthquakes. The trenching revealed downthrown and buried palaeosols, which were dated by radiocarbon. The results of our investigations show that the slip rates are higher than previously thought and that at least four palaeoearthquakes with magnitudes of around M 6.2 occurred during the last 4000–6000 yr. We calculate an average recurrence interval of 1000–1500 yr and a maximum throw rate of ~0.4–0.45 mm a –1 . Based on the new geological earthquake data we developed a seismic hazard scenario, which also incorporates geological site effects. Intensities up to IX must be expected for Northern Attica and the southeastern part of Evia. Earthquake environmental effects like liquefaction and mass movements are also likely to occur. This scenario is in contrast to the official Greek seismic hazard zonation that is based on historical records and assigns different hazard zones for municipalities that will experience the same intensity by earthquakes on the Milesi Fault. We show that the seismic hazard is likely underestimated in our study area and emphasize the need to incorporate geological information in such assessments.

Description: Numerical models of mantle convection typically employ a temperature- or pressure-dependent viscous or viscoplastic rheology and a free slip upper boundary condition. The Earth, however, has a stress-free rather than a free slip surface condition. In addition, with decreasing temperature, the viscosity of rocks increases, which might induce a change from viscous to elastic behaviour (depending on the timescale of deformation). Here, we study the effects of both a Maxwell viscoelastic rheology and a free surface upper boundary condition on viscoelastic convection with a strongly temperature dependent rheology. We particularly focus on the effect of elasticity on the stress state of the lithosphere. Results show that convection vigor and heat transport are not significantly altered by the upper boundary condition or by elasticity. However, the stress state of the lithosphere is significantly affected by both factors. If elasticity is unimportant, a free surface upper boundary condition results in significantly elevated surface stresses (which are up to two magnitudes larger than in the free slip case). Elasticity counteracts this effect and significantly reduces the surface stresses, but distributes stresses over a thicker layer than in the case of a purely viscous rheology. At Earth-like conditions, this effect is significant. While it is warranted to use a free slip upper boundary condition and neglecting elasticity when studying mantle convection and its effect on the thermal state of the Earth, both factors are significant when one wants to predict the stress state of the lithosphere and related questions. Additional 2-D simulations of a plume impinging on a constant thickness and constant viscosity lithosphere show that reasonable parameters might induce lithospheric stress levels that are on the order of a GPa or larger, for viscous free surface models, and that these stresses are several orders of magnitude larger than stresses that occur for free slip models. This suggests that the effect of a free surface should not be ignored in models where the rheology is stress-dependent, such as in viscoplastic models of self-consistent plate tectonics.

Description: On 2012 August 11, a pair of large, damaging earthquakes struck the Varzaghan–Ahar region in northwest Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. To investigate the active tectonics of the source region we applied a combination of seismological methods (local aftershock network, calibrated multiple event relocation and focal mechanism studies), field observations (structural geology and geomorphological) and inversions for the regional stress field. The epicentral region is north of the North Tabriz Fault. The first main shock is characterized by right-lateral strike-slip motion on an almost E–W fault plane of about 23 km length extending from the surface to a depth of about 14 km. The second main shock occurred on an ENE-striking fault that dips at 60–70° to the NW. Independent inversions of focal mechanisms and geologically determined fault kinematic data for the active stress state yield a transpressional tectonic regime with 1 oriented N132E. For the region northeast of the North Tabriz Fault, the presence of rigid lithosphere of the South Caspian Basin implies the kinematic adjustment by northward transferring of the contracted masses through both distributed deformation and structural deflections. Our results suggest that the kinematic adjustment inside a contracting wedge may occur along interacting crosswise or conjugate faults to accommodate low rates of internal deformation. At a global scale, our results indicate that despite the basic assumption of ‘rigid blocks’ in geodetic plate modelling, internal deformation of block-like regions could control the kinematics of deformation and the level of seismic hazard within and around such regions of low deformation rate.

Description: Many salt diapirs are thought to have formed as a result of down-building, which implies that the top of the diapir remained close to the surface during syn-halokinetic sediment deposition. Down-building is largely a 3-D process and in order to better understand what controls the patterns of the diapirs that form by this process, we here perform 3-D numerical models of down-built diapirs initiated by the gravity instability in linear viscous materials and compare the results with analytical models. We vary several parameters of the numerical models such as initial salt thickness, sedimentation rate, salt viscosity, salt-sediment viscosity ratio as well as the density of sediments. Down-building of 3-D diapirs only occurs for a certain range of parameters and is favoured by lower sediment/salt viscosity contrasts and sedimentation rates in agreement with analytical predictions and findings from previous 2-D models. However, the models show that the sedimentation rate has an additional effect on the formation and evolution of 3-D diapir patterns. At low sedimentation rates, salt ridges that form during early model stages remain preserved at later stages as well. For higher sedimentation rates, the initial salt ridges are covered up and finger-like diapirs form at their junctions, which results in different salt exposure patterns at the surface. Once the initial pattern of diapirs is formed, higher sedimentation rate can also result in covered diapirs if the diapir extrusion velocity is insufficiently large. We quantify the effect of sedimentation rate on the number of diapirs exposed at the surface as well as on their spacing and we explain the observations with analytical predictions using thick-plate analytical models. In some cases, this final pattern is distinctly different from the initial polygonal pattern.

Description: We present a generalized formalism for computing gravitationally self-consistent sea level changes driven by the combined effects of dynamic topography, geoid perturbations due to mantle convection, ice mass fluctuations and sediment redistribution on a deforming Earth. Our mathematical treatment conserves mass of the surface (ice plus ocean) load and the solid Earth. Moreover, it takes precise account of shoreline migration and the associated ocean loading. The new formalism avoids a variety of approximations adopted in previous models of sea level change driven by dynamic topography, including the assumption that a spatially fixed isostatic amplification of ‘air-loaded’ dynamic topography accurately accounts for ocean loading effects. While our approach is valid for Earth models of arbitrary complexity, we present numerical results for a set of simple cases in which a pattern of dynamic topography is imposed, the response to surface mass loading assumes that Earth structure varies only with depth and that isostatic equilibrium is maintained at all times. These calculations, involving fluid Love number theory, indicate that the largest errors in previous predictions of sea level change driven by dynamic topography occur in regions of shoreline migration, and thus in the vicinity of most geological markers of ancient sea level. We conclude that a gravitationally self-consistent treatment of long-term sea level change is necessary in any effort to use such geological markers to estimate ancient ice volumes.

Description: The Gibraltar arc and surrounding areas are a complex tectonic region and its tectonic evolution since Miocene is still under debate. Knowledge of its lithospheric structure will help to understand the mechanisms that produced extension and westward motion of the Alboran domain, simultaneously with NW–SE compression driven by Africa–Europe plates convergence. We perform a P -wave receiver function analysis in which we analyse new data recorded at 83 permanent and temporary seismic broad-band stations located in the South of the Iberian peninsula. These data are stacked and combined with data from a previous study in northern Morocco to build maps of thickness and average v P / v S ratio for the crust, and cross-sections to image the lithospheric discontinuities beneath the Gibraltar arc, the Betic and Rif Ranges and their Iberian and Moroccan forelands. Crustal thickness values show strong lateral variations in the southern Iberia peninsula, ranging from ~19 to ~46 km. The Variscan foreland is characterized by a relatively flat Moho at ~31 km depth, and an average v P / v S ratio of ~1.72, similar to other Variscan terranes, which may indicate that part of the lower crustal orogenic root was lost. The thickest crust is found at the contact between the Alboran domain and the External Zones of the Betic Range, while crustal thinning is observed southeastern Iberia (down to 19 km) and in the Guadalquivir basin where the thinning at the Iberian paleomargin could be still preserved. In the cross-sections, we see a strong change between the eastern Betics, where the Iberian crust underthrusts and couples to the Alboran crust, and the western Betics, where the underthrusting Iberian crust becomes partially delaminated and enters into the mantle. The structures largely mirror those on the Moroccan side where a similar detachment was observed in northern Morocco. We attribute a relatively shallow strong negative-polarity discontinuity to the lithosphere-asthenosphere boundary. This means relatively thin lithosphere ranging from ~50 km thickness in southeastern Iberia and northeastern Morocco to ~90–100 km beneath the western Betics and the Rif, with abrupt changes of ~30 km under the central Betics and northern Morocco. Our observations support a geodynamic scenario where in western Betics oceanic subduction has developed into ongoing continental subduction/delamination while in eastern Betics this process is inactive.

Description: Inner core convection, and the corresponding variations in grain size and alignment, has been proposed to explain the complex seismic structure of the inner core, including its anisotropy, lateral variations and the F-layer at the base of the outer core. We develop a parametrized convection model to investigate the possibility of convection in the inner core, focusing on the dominance of the plume mode of convection versus the translation mode. We investigate thermal and compositional convection separately so as to study the end-members of the system. In the thermal case the dominant mode of convection is strongly dependent on the viscosity of the inner core, the magnitude of which is poorly constrained. Furthermore recent estimates of a large core thermal conductivity result in stable thermal stratification, hindering convection. However, an unstable density stratification may arise due to the pressure dependant partition coefficient of certain light elements. We show that this unstable stratification leads to compositionally driven convection, and that inner core translation is likely to be the dominant convective mode due to the low compositional diffusivity. The style of convection resulting from a combination of both thermal and compositional effects is not easy to understand. For reasonable parameter estimates, the stabilizing thermal buoyancy is greater than the destabilizing compositional buoyancy. However we anticipate complex double diffusive processes to occur given the very different thermal and compositional diffusivities.

Description: Relative sea level histories from previously glaciated areas have been used to study Earth rheology and ice sheet evolution during the last glacial cycle. The analysis of postglacial decay times has been used to place estimates on Earth viscosity structure that are relatively independent of uncertainty in the local ice history. Reconstructed sea levels from Ångermanland, Sweden have been commonly adopted for this purpose. We have assessed and compiled an updated relative sea level curve for this region, combining both varve-dated and radiocarbon dated index points. We fitted an exponential curve to the observations, taking into account estimates of eustatic sea level rise, elevation uncertainties as well as the geographical spread of the data sites to arrive at a decay time range (2 ) of 4.2–4.9 kyr for the whole record length (0–8 kyr) and 4.2–6.2 when 0–7 kyr fits are included. We computed model decay times using a large suite of over 900 ice and earth model combinations based on over 400 three-layer Earth viscosity models and more than 30 ice history reconstructions. Based on these extensive results, we confirm that decay time estimates are relatively independent of the regional ice model (at least within the range of ice chronology uncertainties) and so this data parametrization provides a relatively robust measure of Earth viscosity structure. We find that the observational constraints listed above are satisfied by 29 (8 kyr record) and 52 (7 and 8 kyr record) of the viscosity models considered. These subsets define uncertainty ranges in upper and lower mantle viscosity that are interdependent (Fig. 5). Consistent with previous analyses, we find that the observational decay time estimate does not provide useful constraints on model lithospheric thickness (within the range explored; 46–146 km).

Description: Seismological, geological and geodetic data have been integrated to characterize the seismogenic structure of the late 2013-early 2014 moderate energy (maximum local magnitude M Lmax = 4.9) seismic sequence that struck the interior of the Matese Massif, part of the Southern Apennines active extensional belt. The sequence, heralded by a M L = 2.7 foreshock, was characterized by two main shocks with M L = 4.9 and M L = 4.2, respectively, which occurred at a depth of ~17–18 km. The sequence was confined in the 10–20 km depth range, significantly deeper than the 1997–1998 sequence which occurred few km away on the northeastern side of the massif above ~15 km depth. The depth distribution of the 2013–14 sequence is almost continuous, albeit a deeper (16–19 km) and a shallower (11–15 km) group of events can be distinguished, the former including the main shocks and the foreshock. The epicentral distribution formed a ~10 km long NNW–SSE trending alignment, which almost parallels the surface trace of late Pliocene–Quaternary southwest-dipping normal faults with a poor evidence of current geological and geodetic deformation. We built an upper crustal model profile for the eastern Matese massif through integration of geological data, oil exploration well logs and seismic tomographic images. Projection of hypocentres on the profile suggests that the seismogenic volume falls mostly within the crystalline crust and subordinately within the Mesozoic sedimentary cover of Apulia, the underthrust foreland of the Southern Apennines fold and thrust belt. Geological data and the regional macroseismic field of the sequence suggest that the southwest-dipping nodal plane of the main shocks represents the rupture surface that we refer to here as the Matese fault. The major lithological discontinuity between crystalline and sedimentary rocks of Apulia likely confined upward the rupture extent of the Matese fault. Repeated coseismic failure represented by the deeper group of events in the sequence, activated in a passive fashion the overlying ~11–15 km deep section of the upper crustal normal faults. We consider the southwest-dipping Matese fault representative of a poorly known type of seismogenic structures in the Southern Apennines, where extensional seismogenesis and geodetic strain accumulation occur more frequently on NE-dipping, shallower-rooted faults. This is the case of the Boiano Basin fault located on the northern side of the massif, to which the 1997–1998 sequence is related. The close proximity of the two types of seismogenic faults at the Matese Massif is related to the complex crustal architecture generated by the Pliocene–early Pleistocene contractional and transpressional tectonics.

Description: The Scythian Platform (ScP) with a heterogeneous basement of Baikalian–Variscan–Cimmerian age is located between the East European Craton (EEC) on the north and the Crimean–Caucasus orogenic belt and the Black Sea (BS) Basin on the south. In order to get new constrains on the basin architecture and crustal structure of the ScP and a better understanding of the tectonic processes and evolution of the southern margin of the EEC during Mesozoic and Cenozoic time, a 630-km-long seismic wide-angle refraction and reflection (WARR) profile DOBRE-5 was acquired in 2011 October. It crosses in a W–E direction the Fore-Dobrudja Trough, the Odessa Shelf of the BS and the Crimean Plain. The field acquisition included eight chemical shot points located every 50 km and recorded by 215 stations placed every ~2.0 km on the land. In addition, the offshore data from existing profile 26, placed in the Odessa Shelf, were used. The obtained seismic model shows clear lateral segmentation of the crust within the study region on four domains: the Fore-Dobrudja Domain (km 20–160), an offshore domain of the Karkinit Trough at the Odessa Shelf of the BS (km 160–360), an onshore domain of the Central Crimean Uplift (Crimean Plain, km 360–520) and the Indolo-Kuban Trough at the Kerch Peninsula (km 520–620) that is the easternmost part of the Crimea. Two contrasting domains of the ScP within the central part of the DOBRE-5 profile, the Karkinit Trough and the Central Crimean Uplift, may represent different stages of the ScP formation. A deep Karkinit Trough with an underlying high-velocity (〉7.16 km s –1 ) lower crust body suggests its rifting-related origin during Early Cretaceous time. The Central Crimean Uplift represents a thick (up to 47 km) crustal domain consisting of three layers with velocities 5.8–6.4, 6.5–6.6 and 6.7–7.0 km s –1 , which could be evidence of this part of the ScP originating on the crust of Precambrian craton (EEC). The thick heterogeneous basement of the Central Crimean Uplift shows inclusions of granitic bodies associated with magmatic activity related with Variscan orogeny within the ScP. General bending and crustal scale buckling of the Central Crimean Uplift with a wavelength of 230 km could be an effect of the Alpine compressional tectonics in the adjacent Crimean Mountains. The extended/rifted continental margin of the ScP (EEC) at the Odessa Shelf and buckling/uplifted domain of the Central Crimean Uplift affected by compressional tectonics, are separated by the N–S oriented Western Crimean Fault. The crust of the southern margin of the EEC is separated from the ScP, which originated on the EEC crust tectonised and reworked during the Palaeozoic–Mesozoic, by the crustal fault of ~W–E orientation, which corresponds with the Golitsyn Fault observed at the surface between the EEC and the ScP. The Fore-Dobrudja Domain with a thick (〉10 km) heterogeneous basement and two subhorizontal layers in the crystalline crust (with velocities 6.2–6.3 and 6.4–6.65 km s –1 ) differs from the ScP crust and its origin could be very similar to that of the Trans-European Suture Zone and Palaeozoic West European Platform.

Description: New Zealand straddles the boundary between the Australian and Pacific Plate. Cenozoic relative plate motion has resulted in a complex pattern of faulting and block rotation in a zone of continental lithosphere up to 250 km wide. I investigate the implications of the short-term kinematics for the strength of the deforming lithosphere. I use a compilation of seismic reflection/refraction studies and high quality receiver function analyses to determine both the regional structure of the crust, which ranges from 20 to 50 km thick, and fields of buoyancy stress (or GPE per unit volume). Deformation over thousands of years is quantified in terms of velocity and strain rate fields, based on an inversion of neotectonic fault slip and palaeomagnetic data, in the context of the short-term relative plate motions. Forces on the subduction megathrust, as well as deviatoric stresses in the behind subduction region, are calculated from simple 2-D force balances across the Hikurangi Margin, given negligible deviatoric stresses at the along-strike transition between backarc extension and compression. Average megathrust shear stresses are in the range 6–15 MPa, and average lithospheric stresses 〈20 MPa in the overriding plate. The regional lithospheric strength of the plate boundary zone, assuming a viscous rheology (Newtonian or power law), is determined from an inversion of the field of gradients of buoyancy stress (averaged over either the top 25 km of the crust, or 100-km-thick lithosphere) and strain rate, using the thin sheet stress balance equations, calibrated with the subduction force balance analysis. Effective viscosities for the deforming lithosphere and/or crust are in the range 0.1–5 x 10 21 Pa s, with marked weakening in zones of high strain rate, and an abrupt transition to viscosities 〉10 22 Pa s at the margins of the rigid plates. If lateral variations in effective viscosity are only due to non-Newtonian behaviour, these data indicate a bulk power law rheology, with exponent n in the range 2–6. Average lithospheric or crustal deviatoric stresses 〈30 MPa. Such low driving stresses for the deforming crust are likely to be the result of a combination of pore fluid pressures much greater than hydrostatic (〉〉40 per cent lithostatic) and low coefficients of friction (〈〈0.6) on crustal faults.

Description: On 1580 April 6 one of the most destructive earthquakes of northwestern Europe took place in the Dover Strait ( Pas de Calais ). The epicentre of this seismic event, the magnitude of which is estimated to have been about 6.0, has been located in the offshore continuation of the North Artois shear zone, a major Variscan tectonic structure that traverses the Dover Strait. The location of this and two other moderate magnitude historical earthquakes in the Dover Strait suggests that the North Artois shear zone or some of its fault segments may be presently active. In order to investigate the possible fault activity in the epicentral area of the AD 1580 earthquake, we have gathered a large set of bathymetric and seismic-reflection data covering the almost-entire width of the Dover Strait. These data have revealed a broad structural zone comprising several subparallel WNW–ESE trending faults and folds, some of them significantly offsetting the Cretaceous bedrock. The geophysical investigation has also shown some indication of possible Quaternary fault activity. However, this activity only appears to have affected the lowermost layers of the sediment infilling Middle Pleistocene palaeobasins. This indicates that, if these faults have been active since Middle Pleistocene, their slip rates must have been very low. Hence, the AD 1580 earthquake appears to be a very infrequent event in the Dover Strait, representing a good example of the moderate magnitude earthquakes that sometimes occur in plate interiors on faults with unknown historical seismicity.

Description: The normal fault-system responsible of the 2009 M w 6.1 L'Aquila earthquake (Paganica-San Demetrio fault-system) comprises several narrow, fault-parallel valleys of controversial origin. We investigated a key section of the southeastern portion of this fault network along the small Verupola Valley. In order to characterize its nature and possible tectonic activity, we applied multiple-geosciences techniques able to image at depth the structure associated to this peculiar landform. We integrated magnetometry, 2-D P wave and resistivity tomography, surface waves and seismic noise analysis coupled with field mapping, shallow boreholes and trenching. According to our results, the Verupola Valley is a ~30–40-m-deep graben controlled by a SW-dipping master fault and synthetic splays paired with an antithetic NE-dipping fault. The SW-dipping splays are active and cut very shallow (〈2 m deep) Late Pleistocene sediments. The small amount of cumulated vertical offset (~15 m) across the conjugated system may indicate a young fault inception or very low Quaternary slip-rates. Due to its structural continuity with the adjacent mapped strands of the Paganica–San Demetrio fault network, we relate the Verupola Valley to the recent activity of the southeastern segment of this fault system. We also suggest that other fault-parallel valleys can have the same tectonic origin and setting of the Verupola Valley. This latter represents a scale-independent analogue from metric scale (exposed in the palaeoseismological trenches) to the Middle Aterno Basin scale (seen from seismic profiles and fault mapping). Overall, the imaged structural style is coherent with the regional tectonic setting due to Quaternary crustal extension.

Description: We present high-resolution tomographic images in source areas of 26 large crustal earthquakes ( M 6.0–7.2) which occurred in Northeast Japan (Tohoku) during the past 120 yr from 1894 to 2014. Prominent low-velocity (low- V ) and high Poisson's ratio (high- ) anomalies are revealed in the crust and mantle wedge under the source areas. Beneath the volcanic front and backarc areas, the low- V and high- zones reflect arc-magma related high-temperature anomalies which are produced by joint effects of corner flow in the mantle wedge and fluids from dehydration of the subducting Pacific slab. The hot anomalies cause locally thinning and weakening of the brittle seismogenic layer above them. Low-frequency micro-earthquakes are observed in the lower crust and uppermost mantle in or around the low- V zones, which reflect ascending of arc magma and fluids from the mantle wedge to the crust. No volcano and magma exist in the forearc area due to low temperature there, hence the low- V zones in the forearc reflect fluids from the slab dehydration. The ascending fluids may have produced a ‘water wall’ in the mantle wedge and crust beneath the forearc area. When the water enters active faults in the crust, the fault-zone friction is reduced and so large earthquakes can be induced. These results indicate that the nucleation of a large earthquake is not entirely a mechanical process, but is closely associated with subduction dynamics and physical and chemical properties of rocks in the crust and upper mantle. In particular, arc magma and fluids play an important role in the seismogenesis.

Description: For a period of about 1 yr between the summers of 2010 and 2011, 25 broad-band seismographs were deployed in a roughly linear array across the eastern end of the Qaidam basin and the Qilian Shan in the northeastern Tibetan plateau. This region is probably the most suitable place to study the ongoing convergence interaction between the high Tibetan plateau and the main Asian continental plate. Low-frequency P receiver function analysis of the data provides an image of the crust and mantle down to 700 km depth. In addition to the Moho at 45–65 km depth beneath the profile, the 410 and 660 km discontinuities bounding the mantle transition zone can be identified at 400–410 and 650–660 km depths, respectively. A possible increase in temperature in the upper mantle thought to exist beneath the northern part of the high Tibetan plateau is thus confined to this part of the plateau and lower upper-mantle temperatures similar to those beneath southern Tibet occur beneath the Qaidam basin and Qilian Shan. When higher frequencies are included in the P receiver function analysis, a positive Ps converter dipping down to the south from 70–75 km depth at 37.9°N to about 110 km depth at 36°N is imaged. As this feature is only seen in high-frequency images and not in the low-frequency image, it is modelled as the positive Ps conversion from the base of an approximately 5-km-thick anisotropic layer at the top of the Asian mantle lithosphere which is currently subducting. This south-dipping converter continues to the south on the INDEPTH IV profile. S receiver function analysis completes the image of the structure below the Qilian Shan profile with the identification of the lithosphere–asthenosphere boundary (LAB). The LAB of the Asian Plate is identified for a reference slowness of 6.4 s deg –1 at 12–14 s (105–125 km depth) between 38 and 41°N below the northern part of the S receiver function profile. To the south it increases in depth such that it is at about 19 s (170 km depth) between 34 and 35°N at the southern end of the profile. The LAB of the Asian Plate occurs at similar depths on the INDEPTH IV profile at the latitudes where the INDEPTH IV and Qilian Shan profiles overlap. As on the INDEPTH IV profile to the south, between 34 and 35°N at the southern end of the Qilian Shan profile there is evidence from the S receiver functions for the LAB of a separate Tibetan Plate.

Description: Lower and upper bounds for present deformation rates across faults in central California between the San Andreas Fault and Pacific coast are estimated from a new Global Positioning System (GPS) velocity field for central, western California in light of geodetic evidence presented in a companion paper for slow, but significant deformation within the Pacific Plate between young seafloor in the eastern Pacific and older seafloor elsewhere on the plate. Transects of the GPS velocity field across the San Andreas Fault between Parkfield and San Juan Buatista, where fault slip is dominated by creep and the velocity field thus reveals the off-fault deformation, show that GPS sites in westernmost California move approximately parallel to the fault at an average rate of 3.4 ± 0.4 mm yr –1 relative to the older interior of the Pacific Plate, but only 1.8 ± 0.6 mm yr –1 if the Pacific Plate frame of reference is corrected for deformation within the plate. Modelled interseismic elastic deformation from the weakly coupled creeping segment of the San Andreas Fault is an order-of-magnitude too small to explain the southeastward motions of coastal sites in western California. Similarly, models that maximize residual viscoelastic deformation from the 1857 Fort Tejon and 1906 San Francisco earthquakes mismatch both the rates and directions of GPS site motions in central California relative to the Pacific Plate. Neither thus explains the site motions southwest of the San Andreas fault, indicating that the site motions measure deformation across faults and folds outboard of the San Andreas Fault. The non-zero site velocities thus constitute strong evidence for active folding and faulting outboard from the creeping segment of the San Andreas Fault and suggest limits of 0–2 mm yr –1 for the Rinconada Fault slip rate and 1.8 ± 0.6 to 3.4 ± 0.4 mm yr –1 for the slip rates integrated across near-coastal faults such as the Hosgri, San Gregorio and San Simeon faults.

Description: We combine new, well-determined GPS velocities from Clarion, Guadalupe and Socorro islands on young seafloor in the eastern Pacific basin with newly estimated velocities for 26 GPS sites from older seafloor in the central, western and southern parts of the Pacific Plate to test for deformation within the interior of the Pacific Plate and estimate the viscosity of the asthenosphere below the plate. Relative to a Pacific Plate reference frame defined from the velocities of the 26 GPS sites in other areas of the Pacific Plate, GPS sites on Clarion and Guadalupe islands in the eastern Pacific move 1.2 ± 0.6 mm yr –1 (1) towards S09°W ± 38° and 1.9 ± 0.3 mm yr –1 towards S19°E ± 10°, respectively. The two velocities, which are consistent within their 95 per cent uncertainties, both differ significantly from Pacific Plate motion. Transient volcanic deformation related to a 1993–1996 eruption of the Socorro Island shield volcano renders our GPS velocity from that island unreliable for the tectonic analysis although its motion is also southward like those of Clarion and Guadalupe islands. We test but reject the possibilities that drift of Earth's origin in ITRF2008 or unmodelled elastic offsets due to large-magnitude earthquakes around the Pacific rim since 1993 can be invoked to explain the apparent slow southward motions of Clarion and Guadalupe islands. Similarly, corrections to the Pacific Plate GPS velocity field for possible viscoelastic deformation triggered by large-magnitude earthquakes since 1950 also fail to explain the southward motions of the two islands. Viscoelastic models with prescribed asthenospheric viscosities lower than 1  x 10 19 Pa s instead introduce statistically significant inconsistencies into the Pacific Plate velocity field, suggesting that the viscosity of the asthenosphere below the plate is higher than 1  x 10 19 Pa s. Elastic deformation from locked Pacific–North America Plate boundary faults is also too small to explain the southward motions of the two islands. Horizontal thermal contraction of the plate interior may explain the motion observed at Clarion and Guadalupe islands, as might long-term tectonic deformation of the plate interior.

Description: In this study, a new method for computing the sensitivity of the glacial isostatic adjustment (GIA) forward solution with respect to the Earth's mantle viscosity, the so-called the forward sensitivity method (FSM), and a method for computing the gradient of data misfit with respect to viscosity parameters, the so-called adjoint-state method (ASM), are presented. These advanced formal methods complement each other in the inverse modelling of GIA-related observations. When solving this inverse problem, the first step is to calculate the forward sensitivities by the FSM and use them to fix the model parameters that do not affect the forward model solution, as well as identifying and removing redundant parts of the inferred viscosity structure. Once the viscosity model is optimized in view of the forward sensitivities, the minimization of the data misfit with respect to the viscosity parameters can be carried out by a gradient technique which makes use of the ASM. The aim is this paper is to derive the FSM and ASM in the forms that are closely associated with the forward solver of GIA developed by Martinec. Since this method is based on a continuous form of the forward model equations, which are then discretized by spectral and finite elements, we first derive the continuous forms of the FSM and ASM and then discretize them by the spectral and finite elements used in the discretization of the forward model equations. The advantage of this approach is that all three methods (forward, FSM and ASM) have the same matrix of equations and use the same methodology for the implementation of the time evolution of stresses. The only difference between the forward method and the FSM and ASM is that the different numerical differencing schemes for the time evolution of the Maxwell and generalized Maxwell viscous stresses are applied in the respective methods. However, it requires only a little extra computational time for carrying out the FSM and ASM numerically. An straightforward approach to compute the gradient of the data misfit is the brute-force method, whereby the partial derivatives of the misfit with respect to model parameters are approximated by the centred difference of two forward model runs. Although the brute-force method is useful for computing the gradient of the data misfit with respect to a small number of model parameters, it becomes expensive for a viscosity model with a large number of parameters. The ASM offers an efficient alternative for computing the gradient of the misfit since the computational time of the ASM is independent of the number of viscosity parameters. The ASM is thus highly efficient for calculating the gradient of the misfit for models with large numbers of parameters. However, the forward-model solution for each time step must be stored, hence the memory demands scale linearly with the number of time steps. This is the main drawback of the ASM.

Description: Relative to the gravitational potential energy of the Earth's monopole, the multipole energy has received far less attention. In this paper, we recapitulate the basic physics from first principles and derive the formulas for multipole energies in analogy to classical electrostatic theory. We focus on the zonal quadrupole energy associated with the Earth's oblateness, the dominant term in Earth's gravity field apart from the monopole. We find the gravitational energy E oblateness 10 –6 | E monopole | = +2.5 x 10 26 J. We examine the implications of E oblateness and its changes associated with long-term ‘secular’ decreases in the oblateness parameter J 2 . We find the rate of loss of E oblateness due to the Earth rounding induced by the present-day GIA is about –200 GW, an amount quite significant in the kinetic energy budget of the mantle heat engine that drives the plate tectonics that has been estimated to be ~1 TW. We also assert that the tidal braking and the global earthquake dislocations, both resulting in Earth rounding on long-term geological timescales, are accompanied with a secular decrease of E oblateness at nearly the same rate of several GW.

Description: Large-scale chemical lateral heterogeneities are inferred in the Earth's lowermost mantle by seismological studies. We explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time, by using similar analysis in 3-D spherical geometry. In this study, we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle, including the chemical density contrast between the primordial dense material and the regular mantle material (buoyancy ratio), thermal and chemical viscosity contrasts, volume fraction of primordial dense material and the Clapeyron slope of the phase transition at 660 km depth. We find that most of the findings from the 3-D Cartesian study still apply to 3-D spherical cases after slight modifications. Varying buoyancy ratio leads to different flow patterns, from rapid upwelling to stable layering; and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle. Chemical viscosity contrasts in a reasonable range have a second-order role in modifying the stability of the dense anomalies. The volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts, but has significant effects on calculations with intermediate and small thermal viscosity contrasts. The volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast. An endothermic phase transition at 660 km depth acts as a ‘filter’ allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantle.

Description: Relative sea level curves contain coupled information about absolute sea level change and vertical lithospheric movement. Such curves may be constructed based on, for example tide gauge data for the most recent times and different types of geological data for ancient times. Correct account for vertical lithospheric movement is essential for estimation of reliable values of absolute sea level change from relative sea level data and vise versa. For modern times, estimates of vertical lithospheric movement may be constrained by data (e.g. GPS-based measurements), which are independent from the relative sea level data. Similar independent data do not exist for ancient times. The purpose of this study is to test two simple inversion approaches for simultaneous estimation of lithospheric uplift rates and absolute sea level change rates for ancient times in areas where a dense coverage of relative sea level data exists and well-constrained average lithospheric movement values are known from, for example glacial isostatic adjustment (GIA) models. The inversion approaches are tested and used for simultaneous estimation of lithospheric uplift rates and absolute sea level change rates in southwest Scandinavia from modern relative sea level data series that cover the period from 1900 to 2000. In both approaches, a priori information is required to solve the inverse problem. A priori information about the average vertical lithospheric movement in the area of interest is critical for the quality of the obtained results. The two tested inversion schemes result in estimated absolute sea level rise of ~1.2/1.3 mm yr –1 and vertical uplift rates ranging from approximately –1.4/–1.2 mm yr –1 (subsidence) to about 5.0/5.2 mm yr –1 if an a priori value of 1 mm yr –1 is used for the vertical lithospheric movement throughout the study area. In case the studied time interval is broken into two time intervals (before and after 1970), absolute sea level rise values of ~0.8/1.2 mm yr –1 (before 1970) and ~2.0 mm yr –1 (after 1970) are found. The uplift patterns resulting from the different inversions suggest that the lithospheric post-GIA response changes near the border between the Danish Basin and the Fennoscandian Shield. The obtained patterns of vertical lithospheric movement rates are comparable to results from other studies based on different and similar data types. Main differences between the inversion results and the results from other studies are caused by factors such as the simplifications included in the inversion approach, such as neglecting local sea level variation caused by the dominant wind patterns, and the a priori values chosen for the vertical uplift rates. The tests of the inversion schemes reveal that realistic values of absolute sea level rise and lithospheric uplift may be simultaneously estimated provided that reliable prior knowledge regarding the overall lithospheric uplift in the study area is available beforehand. In the presented parametrizations, only one absolute sea level change rate value is estimated for each studied time interval while several vertical movement rates are found, and the inverse estimate of absolute sea level change rate is practically insensitive with respect to the choice of a priori value of absolute sea level change, as long as the uncertainty assigned to this a priori value is kept sufficiently high.

Description: In 1356, a magnitude 6–7 earthquake occurred near Basel, in Switzerland. But recent compilations of GPS measurements reveal that measured horizontal deformation rates in northwestern continental Europe are smaller than error bars on the measurements, proving present tectonic activity, if any, is very small in this area. We propose to reconcile these apparently antinomic observations with a mechanical model of the lithosphere that takes into account the geometry of the lithosphere–asthenosphere boundary, assuming that the only loading mechanism is gravity. The lithosphere is considered to be an elastoplastic material satisfying a Von Mises plasticity criterion. The model, which is 400 km long, 360 km wide and 230 km thick, is centred near Belfort in eastern France, with its width oriented parallel to the N145°E direction. It also takes into account the real topography of both the ground surface and that of the Moho discontinuity. Not only does the model reproduce observed principal stress directions orientations, it also identifies a plastic zone that fits roughly the most seismically active domain of the region. Interestingly, a somewhat similar stress map may be produced by considering an elastic lithosphere and an ad-hoc horizontal ‘tectonic’ stress field. However, for the latter model, examination of the plasticity criterion suggests that plastic deformation should have taken place. It is concluded that the present-day stress field in this region is likely controlled by gravity and rheology, rather than by active Alpine tectonics.

Description: We have investigated variations in transition zone thickness under the Borborema Province of NE Brazil by migrating and stacking teleseismic P -wave receiver functions at 32 seismic stations in the region. The Borborema Province represents the western portion of a larger Neoproterozoic mobile belt that occupied much of northern Gondwana, where extensional processes in the Mesozoic lead to the formation of a number of intracontinental basins and ultimately continental breakup. Episodes of intraplate volcanism and uplift marked the evolution of the Province during the Cenozoic, but it is unclear whether those episodes originated from shallow or deep-seated magmatic sources. On one hand, the elliptical shape of the uplifted area, the stress pattern of the Cenozoic deformation and the time overlap between uplift and volcanism suggest doming from thermal activation due to a deep-seated mantle plume. On the other hand, geochronological dates of volcanic bodies in the Province are better understood if resulting from lithospheric erosion by a shallow, small-scale convection cell. Large temperature anomalies are expected to be associated with mantle upwellings, and constraints on the depth extent of the upwellings can be obtained from transition zone thickness. Thinning of the transition zone with respect to its nominal 250 km value is considered diagnostic for positive temperature anomalies, while thickening is considered diagnostic for negative anomalies. Our results show that transition zone thickness is normal, around 250 km, throughout the Province and suggest that thermal perturbations—if present—are confined to the upper mantle. We argue that our results are consistent with a local, shallow magmatic source for the Cenozoic intraplate volcanism of the Borborema Province, although other proposed scenarios—such as channeling of upwelling plume material along lithospheric thin spots—cannot be ruled out with our analysis.

Description: Standard techniques for computed tomography imaging are not directly applicable to a carbonate rock because of the geometric complexity of its pore space. In this study, we first characterized the pore structure in Majella limestone with 30 per cent porosity. Microtomography data acquired on this rock was partitioned into three distinct domains: macropores, solid grains, and an intermediate domain made up of voxels of solid embedded with micropores below the resolution. A morphological analysis of the microtomography images shows that in Majella limestone both the solid and intermediate domains are interconnected in a manner similar to that reported previously in a less porous limestone. We however show that the macroporosity in Majella limestone is fundamentally different, in that it has a percolative backbone which may contribute significantly to its permeability. We then applied for the first time 3-D-volumetric digital image correlation (DIC) to characterize the mode of mechanical failure in this limestone. Samples were triaxially deformed over a wide range of confining pressures. Tomography imaging was performed on these samples before and after deformation. Inelastic compaction was observed at all tested pressures associated with both brittle and ductile behaviors. Our DIC analysis reveals the structure of compacting shear bands in Majella limestone deformed in the transitional regime. It also indicates an increase of geometric complexity with increasing confinement—from a planar shear band, to a curvilinear band, and ultimately to a diffuse multiplicity of bands, before shear localization is inhibited as the failure mode completes the transition to delocalized cataclastic flow.

Description: Wide-angle reflection/refraction seismic profiles were recorded across the Cyprus Arc, the plate boundary between the African Plate and the Aegean–Anatolian microplate, from the Eratosthenes Seamount to the Hecataeus Rise immediately south of Cyprus. The resultant models were able to resolve detail of significant lateral velocity variations, though the deepest crust and Moho are not well resolved from the seismic data alone. Conclusions from the modelling suggest that (i) Eratosthenes Seamount consists of continental crust but exhibits a laterally variable velocity structure with a thicker middle crust and thinner lower crust to the northeast; (ii) the Hecataeus Rise has a thick sedimentary rock cover on an indeterminate crust (likely continental) and the crust is significantly thinner than Eratosthenes Seamount based on gravity modelling; (iii) high velocity basement blocks, coincident with highs in the magnetic field, occur in the deep water between Eratosthenes and Hecataeus, and are separated and bounded by deep low-velocity troughs and (iv) one of the high velocity blocks runs parallel to the Cyprus Arc, while the other two appear linked based on the magnetic data and run NW–SE, parallel to the margin of the Hecataeus Rise. The high velocity block beneath the edge of Eratosthenes Seamount is interpreted as an older magmatic intrusion while the linked high velocity blocks along Hecataeus Rise are interpreted as deformed remnant Tethyan oceanic crust or mafic intrusives from the NNW–SSE oriented transform margin marking the northern boundary of Eratosthenes Seamount. Eratosthenes Seamount, the northwestern limit of rifted continental crust from the Levant Margin, is part of a jagged rifted margin transected by transform faults on the northern edge of the lower African Plate that is being obliquely subducted under the Aegean–Anatolian upper plate. The thicker crust of Eratosthenes Seamount may be acting as an asperity on the subducting slab, locally locking up subduction of the Cyprus Arc on its northern margin, while deformed Tethyan oceanic crust remains trapped between its northeastern margin and the Hecataeus Rise.

Description: We present an up-to-date high resolution picture of the ongoing crustal deformation field of Italy, based on an extensive combination of permanent and non-permanent GPS observations carried out since 1994. In addition, we present an updated map of contemporary S Hmax orientations computed by a multidisciplinary data set of well-constrained stress indicators, including both published results and novel analyses. The comparison of stress and geodetic strain-rates directions reveals that both patterns are near-parallel over a large part of the investigated area, highlighting that crustal stress and surface deformation are driven by the same mechanism. The comparison of the azimuthal patterns of surface strain and mantle deformation shows a modest correlation on the Alps and a low correlation along the Apennines chain and the Calabro-Peloritan Arc. Along the Apennines chain, this feature suggests the occurrence of significant strain partitioning and crust–mantle mechanical decoupling. Along the Calabro-Peloritan Arc, the apparent low correlation reflects a different mantle–crust mechanism of deformation to the ongoing subduction and rollback of the Ionian slab. In addition, the superposition of regional/local effects related to second-order sources (crustal lateral density changes, strength contrasts), which at regional/local scale modulate the crustal stress/strain-rate pattern, cannot be ruled out.

Description: A series of linear analysis was performed on the onset of thermal convection of highly compressible fluids, in order to deepen the fundamental insights into the mantle convection of massive super-Earths in the presence of strong adiabatic compression. We consider the temporal evolution (growth or decay) of an infinitesimal perturbation superimposed to a highly compressible fluid which is in a hydrostatic (motionless) and conductive state in a basally heated horizontal layer. As a model of pressure-dependence in material properties, we employed an exponential decrease in thermal expansivity α and exponential increase in (reference) density with depth. The linearized equations for conservation of mass, momentum and internal (thermal) energy are numerically solved for the critical Rayleigh number as well as the vertical profiles of eigenfunctions for infinitesimal perturbations. The above calculations are repeatedly carried out by systematically varying (i) the dissipation number (Di), (ii) the temperature at the top surface and (iii) the magnitude of pressure-dependence in α and . Our analysis demonstrated that the onset of thermal convection is strongly affected by the adiabatic compression, in response to the changes in the static stability of thermal stratification in the fluid layer. For sufficiently large Di where a thick sublayer of stable stratification develops in the layer, for example, the critical Rayleigh number explosively increases with Di, together with drastic decreases in the length scales of perturbations both in vertical and horizontal directions. In particular, for very large Di, a thick ‘stratosphere’ occurs in the fluid layer where the vertical motion is significantly suppressed, resulting in a shrink of the incipient convection in a thin sublayer of unstable thermal stratification. In addition, when Di exceeds a threshold value above which a thermal stratification becomes stable in the entire layer, no perturbation is allowed to grow with time regardless of the Rayleigh number and/or the horizontal wavelength. We also found that the effect of adiabatic compression becomes prominent for higher temperature at the top surface of the fluid layer. These findings may imply the crucial importance of adiabatic compression in understanding the dynamics and evolution of the mantles of massive super-Earths, particularly for those orbiting their parent stars very closely.

Description: In this paper, we propose an approach to compute the coseismic Earth's volume change based on a spherical-Earth elastic dislocation theory. We present a general expression of the Earth's volume change for three typical dislocations: the shear, tensile and explosion sources. We conduct a case study for the 2004 Sumatra earthquake ( M w 9.3), the 2010 Chile earthquake ( M w 8.8), the 2011 Tohoku-Oki earthquake ( M w 9.0) and the 2013 Okhotsk Sea earthquake ( M w 8.3). The results show that mega-thrust earthquakes make the Earth expand and earthquakes along a normal fault make the Earth contract. We compare the volume changes computed for finite fault models and a point source of the 2011 Tohoku-Oki earthquake ( M w 9.0). The big difference of the results indicates that the coseismic changes in the Earth's volume (or the mean radius) are strongly dependent on the earthquakes’ focal mechanism, especially the depth and the dip angle. Then we estimate the cumulative volume changes by historical earthquakes ( M w ≥ 7.0) since 1960, and obtain an Earth mean radius expanding rate about 0.011 mm yr –1 .

Description: We present the crustal resistivity structure of the Pamir and Southern Tian Shan orogenic belts at the northwestern promontory of the India–Asia collision zone. The magnetotelluric (MT) data were recorded along a roughly north–south trending, 350 km long corridor from the Pamir Plateau in southern Tajikistan across the Pamir frontal ranges, the Alai Valley and the southwestern Tian Shan to Osh in the Kyrgyz part of the Fergana Basin. In total, we measured at 178 sites, whereof 26 combine broad band and long period recordings. One of the most intriguing features of the 2-D and 3-D inversion results is a laterally extended zone of high electrical conductivity below the Pamir Plateau, with resistivities below 1 m, starting at a depth of ~10–15 km. The high conductivity can be explained with the presence of partially molten rocks at middle to lower crustal levels, possibly related to ongoing migmatization and/or middle/lower crustal flow underneath the Southern Pamir. This interpretation is consistent with a low velocity zone found from local earthquake tomography, relatively high v p / v s ratios, elevated surface heat flow, and thermomechanical modelling suggesting that melting temperatures are reached in the felsic middle crust. In the upper crust of the Pamir and Tian Shan, the Palaeozoic–Mesozoic suture zones appear as electrically conductive, whereas the compact metamorphic rocks of the Muskol-Shatput Dome of the Central Pamir are highly resistive. The intra-montane basin of the Alai Valley—sandwiched between the Pamir and Tian Shan—exhibits a generally conductive upper crust that bifurcates into two conductors at depth. One of them connects to the active Main Pamir Thrust, which is absorbing most of today's convergence between the Pamir and the Tian Shan. Several deeper zones of high conductivity in the middle and lower crust of Central and Northern Pamir likely record fluid release due to metamorphism associated with active continental subduction/delamination.

Description: A new model of the deglaciation history of Antarctica over the past 25 kyr has been developed, which we refer to herein as ICE-6G_C (VM5a). This revision of its predecessor ICE-5G (VM2) has been constrained to fit all available geological and geodetic observations, consisting of: (1) the present day uplift rates at 42 sites estimated from GPS measurements, (2) ice thickness change at 62 locations estimated from exposure-age dating, (3) Holocene relative sea level histories from 12 locations estimated on the basis of radiocarbon dating and (4) age of the onset of marine sedimentation at nine locations along the Antarctic shelf also estimated on the basis of 14 C dating. Our new model fits the totality of these data well. An additional nine GPS-determined site velocities are also estimated for locations known to be influenced by modern ice loss from the Pine Island Bay and Northern Antarctic Peninsula regions. At the 42 locations not influenced by modern ice loss, the quality of the fit of postglacial rebound model ICE-6G_C (VM5A) is characterized by a weighted root mean square residual of 0.9 mm yr –1 . The Southern Antarctic Peninsula is inferred to be rising at 2 mm yr –1 , requiring there to be less Holocene ice loss there than in the prior model ICE-5G (VM2). The East Antarctica coast is rising at approximately 1 mm yr –1 , requiring ice loss from this region to have been small since Last Glacial Maximum. The Ellsworth Mountains, at the base of the Antarctic Peninsula, are inferred to be rising at 5–8 mm yr –1 , indicating large ice loss from this area during deglaciation that is poorly sampled by geological data. Horizontal deformation of the Antarctic Plate is minor with two exceptions. First, O'Higgins, at the tip of the Antarctic Peninsula, is moving southeast at a significant 2 mm yr –1 relative to the Antarctic Plate. Secondly, the margins of the Ronne and Ross Ice Shelves are moving horizontally away from the shelf centres at an approximate rate of 0.8 mm yr –1 , in viscous response to the early Holocene unloading of ice from the current locations of the ice shelf centers. ICE-6G_C (VM5A) fits the horizontal observations well (wrms residual speed of 0.7 mm yr –1 ), there being no need to invoke any influence of lateral variation in mantle viscosity. ICE-6G_C (VM5A) differs in several respects from the recently published W12A model of Whitehouse et al. First, the upper-mantle viscosity in VM5a is 5 10 20 Pa s, half that in W12A. The VM5a profile, which is identical to that inferred on the basis of the Fennoscandian relaxation spectrum, North American relative sea level histories and Earth rotation constraints, when coupled with the revised ICE-6G_C deglaciation history, fits all of the available constraints. Secondly, the net contribution of Antarctica ice loss to global sea level rise is 13.6 m, 2/3 greater than the 8 m in W12A. Thirdly, ice loss occurs quickly from 12 to 5 ka, and the contribution to global sea level rise during Meltwater Pulse 1B (11.5 ka) is large (5 m), consistent with sedimentation constraints from cores from the Antarctica ice shelf. Fourthly, in ICE-6G_C there is no ice gain in the East Antarctica interior, as there is in W12A. Finally, the new model of Antarctic deglaciation reconciles the global constraint upon the global mass loss during deglaciation provided by the Barbados record of relative sea level history when coupled with the Northern Hemisphere counterpart of this new model.

Description: The Himalaya is the result of the on-going convergence and collision of India and Asia. The internal configuration and processes that govern the rise of the Himalayan Mountains and Tibetan Plateau are crucial to understand continental collision zones. However, knowledge of the prior configuration of the colliding plates is equally important, since inherited (pre-orogenic/basement) structures can undeniably influence the development of the orogenic architecture throughout the orogen's cycle of collision and eventual collapse. Three northeast-trending palaeotopographic ridges of faulted Precambrian Indian basement underlie the Ganga basin south of the Himalaya. Our paper illustrates a crustal-scale fault origin for these ridges and succeeds in determining how far north beneath the Himalayan system they extend and how they ultimately govern the location of upper crustal faults in southern Tibet. Spectrally filtered EGM2008 Bouguer gravity data and edges in its horizontal gradient at different source depths (‘gravity worms’) over northern Peninsular India, the Himalaya and southern Tibet reveal several continuous Himalayan cross-strike discontinuities interpreted to represent crustal faults. Gravity lineaments in Peninsular India coincide with edges of the Precambrian basement ridges and megakinks up to 100 km wide develop in foreland cover sequences between the interpreted basement faults. The interpreted basement faults project northward beneath the Himalayan system and southern Tibet. Our results suggest that several active Himalayan cross-strike faults, such as the ones related to many graben in southern Tibet, are rooted in the underplated Indian lower crust or step en échelon along interpreted basement faults. Our interpretation thus suggests that south Tibet graben are spatially related to deep-seated crustal-scale faults rooted in the underplated Indian crust. These major discontinuities partition the Himalayan range into distinct zones, and could ultimately contribute to lateral variability in tectonic evolution along the orogen's strike.

Description: We explore the impact of deep ductile shear zones on post-seismic deformation following a finite length strike-slip earthquake. We show that the pattern of post-seismic vertical surface deformation surrounding the fault is a discriminant for the existence of high viscosities immediately below the seismogenic layer, regardless of whether the model contains purely distributed creep or also includes a component of localized creep at subseismogenic depths. Post-seismic deformation characterized by initially fast relaxation followed by a slower relaxation is predicted by models that include both localized creep in a subseismogenic shear zone and distributed creep in the surrounding region, even if they only contain steady Maxwell viscoelasticity. This post-seismic deformation is similar to that in models that approximate the ductile lithosphere and/or asthenosphere with Burgers viscoelasticity. We find that the post-seismic deformation following the 1997 M w 7.6 Manyi, China, earthquake, is consistent with a post-seismic model composed of a lower Maxwell viscoelastic region with viscosity 10 19 Pa s and a 5 km wide, Maxwell viscoelastic shear zone with viscosity 10 18 Pa s beneath the fault.

Description: The Amundsen Sea Embayment of West Antarctica represents a key component in the tectonic history of Antarctic–New Zealand continental breakup. The region played a major role in the plate-kinematic development of the southern Pacific from the inferred collision of the Hikurangi Plateau with the Gondwana subduction margin at approximately 110–100 Ma to the evolution of the West Antarctic Rift System. However, little is known about the crustal architecture and the tectonic processes creating the embayment. During two ‘RV Polarstern’ expeditions in 2006 and 2010 a large geophysical data set was collected consisting of seismic-refraction and reflection data, ship-borne gravity and helicopter-borne magnetic measurements. Two P -wave velocity–depth models based on forward traveltime modelling of nine ocean bottom hydrophone recordings provide an insight into the lithospheric structure beneath the Amundsen Sea Embayment. Seismic-reflection data image the sedimentary architecture and the top-of-basement. The seismic data provide constraints for 2-D gravity modelling, which supports and complements P -wave modelling. Our final model shows 10–14-km-thick stretched continental crust at the continental rise that thickens to as much as 28 km beneath the inner shelf. The homogenous crustal architecture of the continental rise, including horst and graben structures are interpreted as indicating that wide-mode rifting affected the entire region. We observe a high-velocity layer of variable thickness beneath the margin and related it, contrary to other ‘normal volcanic type margins’, to a proposed magma flow along the base of the crust from beneath eastern Marie Byrd Land—West Antarctica to the Marie Byrd Seamount province. Furthermore, we discuss the possibility of upper mantle serpentinization by seawater penetration at the Marie Byrd Seamount province. Hints of seaward-dipping reflectors indicate some degree of volcanism in the area after break-up. A set of gravity anomaly data indicate several phases of fully developed and failed rift systems, including a possible branch of the West Antarctic Rift System in the Amundsen Sea Embayment.

Description: Geophysical data are the main source of information about the subsurface. Geophysical techniques are, however, highly non-unique in determining specific physical parameters and boundaries of subsurface objects. To obtain actual physical information, an inversion process is often applied, in which measurements at or above the Earth surface are inverted into a 2- or 3-D subsurface spatial distribution of the physical property. Interpreting these models into structural objects, related to physical processes, requires a priori knowledge and expert analysis which is susceptible to subjective choices and is therefore often non-repeatable. In this research, we implemented a recently introduced object-based approach to interpret the 3-D inversion results of a single geophysical technique using the available a priori information and the physical and geometrical characteristics of the interpreted objects. The introduced methodology is semi-automatic and repeatable, and allows the extraction of subsurface structures using 3-D object-oriented image analysis (3-D OOA) in an objective knowledge–based classification scheme. The approach allows for a semi-objective setting of thresholds that can be tested and, if necessary, changed in a very fast and efficient way. These changes require only changing the thresholds used in a so-called ruleset, which is composed of algorithms that extract objects from a 3-D data cube. The approach is tested on a synthetic model, which is based on a priori knowledge on objects present in the study area (Tanzania). Object characteristics and thresholds were well defined in a 3-D histogram of velocity versus depth, and objects were fully retrieved. The real model results showed how 3-D OOA can deal with realistic 3-D subsurface conditions in which the boundaries become fuzzy, the object extensions become unclear and the model characteristics vary with depth due to the different physical conditions. As expected, the 3-D histogram of the real data was substantially more complex. Still, the 3-D OOA-derived objects were extracted based on their velocity and their depth location. Spatially defined boundaries, based on physical variations, can improve the modelling with spatially dependent parameter information. With 3-D OOA, the non-uniqueness on the location of objects and their physical properties can be potentially significantly reduced.

Description: We estimate Eurasia-North America Plate motion rotations at ~1-Myr intervals for the past 20 Myr from more than 11 000 crossings of 21 magnetic reversals from Chron 1n (0.78 Ma) to C6no (19.72 Ma) and flow lines digitized from the Charlie Gibbs, Bight and Molloy fracture zones and transform faults. Adjusted for outward displacement, the 21 best-fitting rotations determined from a simultaneous inversion of the numerous kinematic data reconstruct the reversal crossings with weighted root mean square misfits of only 1–2 km and 0.2–7 km for the transform fault and fracture zone crossings. The new rotations clearly define a ~1000 km southward shift of the rotation pole and 20 per cent slowdown in seafloor spreading rates between 7 and 6 Ma, preceded by apparently steady plate motion from 19.7 to ~7 Ma. Data for times since C3An.2 (6.7 Ma) are well fit by a stationary pole of rotation and constant rate of angular opening, consistent with steady motion since 6.7 Ma. The southward shift of the rotation pole at 7–6 Ma implies that Eurasia-North America motion in northeastern Asia changed from slowly convergent before 7 Ma to slowly divergent afterward. Crossings of magnetic reversals C1n through C3An.1 (6.0 Ma) are well fit everywhere in the Arctic basin and south to the Azores triple junction, indicating that the Eurasia and North America plates have not deformed along their mutual boundary since at least 6.0 Ma. However, the new rotations systematically overrotate magnetic lineations older than C3An.1 (6.0 Ma) within 200 km of the Azores triple junction and also overrotate lineations older than C5n along the Gakkel Ridge in the Arctic Basin. Barring misidentifications of the magnetic anomalies in those areas, the pattern and magnitude of the systematic misfits imply that slow (~1 mm yr –1 ) distributed or microplate deformation occurred in one or both regions.

Description: Here we inverted the GPS data to infer the coseismic slip of the Tohoku-Oki earthquake and the time-dependent afterslip distribution in the 4 months following the main shock. The Tohoku-Oki earthquake showed an unexpected magnitude and a characteristic depth-dependent differentiation of seismic energy radiation. In this context the estimation and comparison of the distribution of the fault portions that slip coseismically and post-seismically contribute to a better understanding of the variation of frictional characteristics of the plate interface. The inferred coseismic slip extends in a relatively compact region located updip from the hypocentre and reaches its highest value (about 60 m) near the trench. Afterslip occurs mostly outside the coseismic rupture and is distributed in two main modal centres. It reaches its largest values in an area located downdip of the coseismic slip and extends to a depth of 80 km. In the depth range between 30 and 50 km afterslip overlaps the portion of the fault that experienced historical moderate earthquakes, high-frequency seismic radiation and thrust-type aftershocks. The behaviour of this area can be explained by a rheologically heterogeneous region made of a ductile fault matrix interspersed with compact brittle asperities. On the contrary, the region beneath 50–60 km depth is probably characterized by a fully velocity strengthening behaviour. Southern afterslip, located off-Chiba Prefecture, is probably related to the M w 7.9 Ibaraki-Oki aftershock. The northward extension of the afterslip stops at a latitude of about 40°N, just south of the off-Aomori region. This may be related to three large events occurred in this area during the last century and the consequent strong coupling or complete depletion of the accumulated strain that characterize this region.

Description: We present a revised interpretation of magnetic anomalies and fracture zones on the Southwest Indian Ridge (SWIR; Africa–Antarctica) and the Southeast Indian Ridge (SEIR; Capricorn–Antarctica) and use them to calculate 2-plate finite rotations for anomalies 34 to 20 (84 to 43 Ma). Central Indian Ridge (CIR; Capricorn–Africa) rotations are calculated by summing the SWIR and SEIR rotations. These rotations provide a high-resolution record of changes in the motion of India and Africa at the time of the onset of the Reunion plume head. An analysis of the relative velocities of India, Africa and Antarctica leads to a refinement of previous observations that the speedup of India relative to the mantle was accompanied by a slowdown of Africa. The most rapid slowdown of Africa occurs around Chron 32Ay (71 Ma), the time when India's motion relative to Africa notably starts to accelerate. Using the most recent Geomagnetic Polarity Timescale (GTS12) we show that India's velocity relative to Africa was characterized by an acceleration from roughly 60 to 180 mm yr –1 between 71 and 66 Ma, a short pulse of superfast motion (~180 mm yr –1 ) between 66 and 63 Ma, an abrupt slowdown to 120 mm yr –1 between 63 and 62 Ma, and then a long period (63 to 47 Ma) of gradual slowing, but still fast motion (~100 mm yr –1 ), which ends with a rapid slowdown after Chron 21o (47 Ma). Changes in the velocities of Africa and India with respect to the mantle follow a similar pattern. The fastest motion of India relative to the mantle, ~220 mm yr –1 , occurs during Chron 29R. The SWIR rotations constrain three significant changes in the migration path of the Africa–Antarctic stage poles: following Chron 33y (73 Ma), following Chron 31y (68 Ma), and following Chron 24o (54 Ma). The change in the migration path of the SWIR stage poles following Chron 33y is coincident with the most rapid slowdown in Africa's motion. The change in the migration path after Chron 31y, although coincident with the most rapid acceleration of India's northward motion, may be related to changes in ridge push forces on the SWIR associated with the onset of extension along the Bain transform fault zone. The initial slowdown in India's motion relative to Africa between 63 and 62 Ma is more abrupt than predictions based on published plume head force models, suggesting it might have been caused by a change in plate boundary forces. The abrupt change in the migration path of the SWIR stage poles after Chron 24o is not associated with major changes in the velocities of either Africa or India and may reflect Atlantic basin plate motion changes associated with the arrival at the Earth's surface of the Iceland plume head. The abruptness of India's slowdown after Chron 21o is consistent with a collision event.

Description: Thin plate flexure theory provides an accurate model for the response of the lithosphere to vertical loads on horizontal length scales ranging from tens to hundreds of kilometres. Examples include flexure at seamounts, fracture zones, sedimentary basins and subduction zones. When applying this theory to real world situations, most studies assume a locally uniform plate thickness to enable simple Fourier transform solutions. However, in cases where the amplitude of the flexure is prominent, such as subduction zones, or there are rapid variations in seafloor age, such as fracture zones, these models are inadequate. Here we present a computationally efficient algorithm for solving the thin plate flexure equation for non-uniform plate thickness and arbitrary vertical load. The iterative scheme takes advantage of the 2-D fast Fourier transform to perform calculations in both the spatial and spectral domains, resulting in an accurate and computationally efficient solution. We illustrate the accuracy of the method through comparisons with known analytic solutions. Finally, we present results from three simple models demonstrating the differences in trench outer rise flexure when 2-D variations in plate rigidity and applied bending moment are taken into account. Although we focus our analysis on ocean trench flexure, the method is applicable to other 2-D flexure problems having spatial rigidity variations such as seamount loading of a thermally eroded lithosphere or flexure across the continental–oceanic crust boundary.

Description: Theory has been long established for computing the elastic response of a spherically symmetric terrestrial planetary body to both body tide and surface loading forces. However, for a planet with laterally heterogeneous mantle structure, the response is usually computed using a fully numerical approach. In this paper, we develop a semi-analytic method based on perturbation theory to solve for the elastic response of a planetary body with lateral heterogeneities in its mantle. We present a derivation of the governing equations for our second-order perturbation method and use them to study the high-order tidal effects caused by mode coupling between degree-2 body tide forcing and the laterally heterogeneous elastic structure of the mantle. We test our method by applying it to the Moon in which small long-wavelength lateral heterogeneities are assumed to exist in the elastic moduli of the lunar mantle. The tidal response of the Moon is determined mode by mode, for lateral heterogeneities with different depth ranges within the mantle and different horizontal scales. Our perturbation method solutions are compared with numerical results, showing remarkable agreement between the two methods. We conclude that our perturbation method provides accurate results and can be adapted to address a variety of forward and inverse response problems.

Description: Teleseismic data recorded at 13 broad-band seismological stations across northwest part of the Tethyan Himalaya and eastern Ladakh are analysed to determine the seismic characteristics of the crust and upper mantle beneath the northwest India–Asia collision zone. The receiver functions computed from teleseismic P- waveform for a wide range of backazimuth show strong azimuthal variation in the Indus suture zone (ISZ), the zone which marks the collision and subsequent subduction of both the Tethyan oceanic plate and Indian continental plate beneath Eurasia. The teleseismic waves piercing the ISZ do not show clear P -to- S ( Ps ) converted phase at the depth of Moho. In contrast, the waves piercing the Karakoram zone, Ladakh batholith and the Tethyan Himalayan region south of the ISZ clearly show the Moho converted Ps phase and corresponding inverted models reveal variation of crustal thickness from ~60 km beneath the Tethyan Himalaya to ~80 km beneath the Karakoram fault zone. A prominent intracrustal low velocity zone (IC-LVZ) is detected in the shear wave velocity models within the depth range ~15–40 km. The IC-LVZ identified at the stations both north and south of the ISZ can be interpreted as due to presence of fluid/partial melt. Our study provides compelling evidence that the mid-crustal low velocity zone does extend across the suture zone, in to the Tethyan Himalaya. The contact between this serpentinized ultramafic rocks and the eclogitized Indian continental crust in the suture zone is identified at ~47–50 km depth.

Description: The Tien Shan is the largest active intracontinental orogenic belt on Earth. To better understand the processes causing mountains to form at great distances from a plate boundary, we analyse passive source seismic data collected on 40 broad-band stations of the MANAS project (2005–2007) and 12 stations of the permanent KRNET seismic network to determine variations in crustal thickness and shear wave speed across the range. We jointly invert P - and S -wave receiver functions with surface wave observations from both earthquakes and ambient noise to reduce the ambiguity inherent in the images obtained from the techniques applied individually. Inclusion of ambient noise data improves constraints on the upper crust by allowing dispersion measurements to be made at shorter periods. Joint inversion can also reduce the ambiguity in interpretation by revealing the extent to which various features in the receiver functions are amplified or eliminated by interference from multiples. The resulting wave speed model shows a variation in crustal thickness across the range. We find that crustal velocities extend to ~75 km beneath the Kokshaal Range, which we attribute to underthrusting of the Tarim Basin beneath the southern Tien Shan. This result supports the plate model of intracontinental convergence. Crustal thickness elsewhere beneath the range is about 50 km, including beneath the Naryn Valley in the central Tien Shan where previous studies reported a shallow Moho. This difference apparently is the result of wave speed variations in the upper crust that were not previously taken into account. Finally, a high velocity lid appears in the upper mantle of the Central and Northern part of the Tien Shan, which we interpret as a remnant of material that may have delaminated elsewhere under the range.

Description: Errors in the satellite orbits are considered to be a limitation for Interferometric Synthetic Aperture Radar (InSAR) time-series techniques to accurately measure long-wavelength (〉50 km) ground displacements. Here we examine how orbital errors propagate into relative InSAR line-of-sight velocity fields and evaluate the contribution of orbital errors to the InSAR uncertainty. We express the InSAR uncertainty due to the orbital errors in terms of the standard deviations of the velocity gradients in range and azimuth directions (range and azimuth uncertainties). The range uncertainty depends on the magnitude of the orbital errors, the number and time span of acquisitions. Using reported orbital uncertainties we find range uncertainties of less than 1.5 mm yr –1  100 km –1 for ERS, less than 0.5 mm yr –1  100 km –1 for Envisat and ~0.2 mm yr –1  100 km –1 for TerraSAR-X and Sentinel-1. Under a conservative scenario, we find azimuth uncertainties of better than 1.5 mm yr –1  100 km –1 for older satellites (ERS and Envisat) and better than 0.5 mm yr –1  100 km –1 for modern satellites (TerraSAR-X and Sentinel-1). We validate the expected uncertainties using LOS velocity fields obtained from Envisat SAR imagery. We find residual gradients of 0.8 mm yr –1  100 km –1 or less in range and of 0.95 mm yr –1  100 km –1 or less in azimuth direction, which fall within the 1 to 2 uncertainties. The InSAR uncertainties due to the orbital errors are significantly smaller than generally expected. This shows the potential of InSAR systems to constrain long-wavelength geodynamic processes, such as continent-scale deformation across entire plate boundary zones.

Description: A method for subsurface recognition of blind geological bodies is presented using combined surface constraints and 3-D structural modelling that incorporates constraints from detailed mapping, and potential-field inversion modelling. This method is applied to the Mount Painter Province and demonstrates that addition of low density material is required to reconcile the gravity signature of the region. This method may be an effective way to construct 3-D models in regions of excellent structural control, and can be used to assess the validity of surface structures with 3-D architecture. Combined geological and potential-field constrained inversion modelling of the Mount Painter Province was conducted to assess the validity of the geological models of the region. Magnetic susceptibility constrained stochastic property inversions indicates that the northeast to southwest structural trend of the relatively magnetic meta-sedimentary rocks of the Radium Creek Group in the Mount Painter Inlier is reconcilable with the similar, northeast to southwest trending positive magnetic anomalies in the region. Radium Creek Group packages are the major contributor of the total magnetic response of the region. However field mapping and the results of initial density constrained stochastic property inversion modelling do not correlate with a large residual negative gravity anomaly central to the region. Further density constrained inversion modelling indicates that an additional large body of relatively low density material is needed within the model space to account for this negative density anomaly. Through sensitivity analysis of multiple geometrical and varied potential-field property inversions, the best-fitting model records a reduction in gravity rms misfit from 21.9 to 1.69 mGal, representing a reduction from 56 to 4.5 per cent in respect to the total dynamic range of 37.5 mGal of the residual anomaly. This best-fitting model incorporates a volumetrically significant source body of interpreted felsic, low density material (10 12 m 3 ) impinging on the central-west of the Mount Painter Inlier and overlying Neoproterozoic sequences, and the emplacement of more mafic affinities in the northeast and east. The spatial association and circular geometry of these granitoid bodies suggests an affinity with the Palaeozoic ~460–440 Ma British Empire Granite that outcrops in the Mount Painter Inlier. The intrusion of this additional material in the Palaeozoic could either be the product of; or contributed to, an increased local geotherm and heat flow in the region during the Palaeozoic.

Description: The Pacific and Australian plates in the South Island, New Zealand (NZ) converge at a rate of about 4 cm yr –1 . Accommodation of the continental part of this convergence in the lithospheric mantle is both poorly understood and currently controversial yet it is a problem of fundamental importance for understanding lithospheric thickening. End-member possibilities range from the classical model of asymmetric subduction to symmetric viscous thickening. Seismic tomography has the potential to image this process. However, tomographic images to date are poorly constrained due to the lack of appropriate earthquakes. Improved teleseismic tomography of the region has been achieved by increasing data coverage and applying a novel scheme of correcting for crustal structure by ray tracing through a newly created model of shallow shear wave velocity derived from the inversion of noise-based dispersion measurements. Our resulting models suggest the lithospheric mantle high velocities at the continental plate boundary extend no deeper than approximately 125 km, evidence against both previous models of viscous drip and typical asymmetric subduction zones. This high velocity core extends from north to south along the axis of South Island suggesting that mantle convergence is accommodated along the older, mid-Cenozoic, plate boundary. West of South Island, a high velocity west dipping zone may define the remnant Cretaceous subduction zone that has been distorted by Cenozoic transcurrent deformation. We present our new 3-D seismic velocity models together with a compatible tectonic model and discuss their implications for the nature of lithospheric evolution at this convergent boundary.

Description: We use continuous GPS measurements from 31 stations in southern Mexico to model coseismic slip and post-seismic deformation from the 2012 March 20 M w  = 7.5 Ometepec earthquake, the first large thrust earthquake to occur below central Mexico during the modern GPS era. Coseismic offsets ranging from ~280 mm near the epicentre to 5 mm or less at sites far from the epicentre are fit best by a rupture focused between ~15 and 35 km depth, consistent with an independent seismological estimate. The corresponding geodetic moment of 1.4 10 20 N·m is within 10 per cent of two independent seismic estimates. Transient post-seismic motion recorded by GPS sites as far as 300 km from the rupture has a different horizontal deformation gradient and opposite sense of vertical motion than do the coseismic offsets. A forward model of viscoelastic relaxation as a result of our new coseismic slip solution incorrectly predicts uplift in areas where post-seismic subsidence was recorded and indicates that viscoelastic deformation was no more than a few per cent of the measured post-seismic deformation. The deformation within 6 months of the earthquake was thus strongly dominated by fault afterslip. The post-seismic GPS time-series are well fit as logarithmically decaying fault afterslip on an area of the subduction interface up to 10 times larger than the earthquake rupture zone, extending as far as 220 km inland. Afterslip had a cumulative geodetic moment of 2.0 10 20 N·m, ~40 per cent larger than the Ometepec earthquake. Tests for the shallow and deep limits for the afterslip require that it included much of the earthquake rupture zone as well as regions of the subduction interface where slow slip events and non-volcanic tremor have been recorded and areas even farther downdip on the flat interface. Widespread afterslip below much of central Mexico suggests that most of the nearly flat subduction interface in this region is conditionally stable and thus contributes measurable transient deformation to large areas of Mexico south of and in the volcanic belt.

Description: We have derived a shallow subsurface 2-D tomographic P -wave velocity image of the Deccan Volcanic Province (DVP) of India using first-arrival traveltime data along a 90-km-long N–S trending seismic profile in the Deccan Syneclise region. The tomographic image depicts smooth velocity variations of Quaternary and Tertiary (2.0–3.0 km s –1 ) sediments, basalts/traps (5.0–5.5 km s –1 ), sub-trappean Mesozoic sediments (4.3–4.5 km s –1 ) as well as the basement (5.9–6.1 km s –1 ) geometry down to a maximum depth of 5.0 km. Due to Late Cretaceous volcanism and outpouring of basaltic lava flows, this region is affected by numerous dyke intrusions and thick basaltic trap (2–3 km) exposed on the surface and surrounded by graben structures due to deep basinal faults forming a large igneous province. Although sub-basalt imaging is a major challenge for the oil industry, with the help of tomographic imaging technique of first-arrival seismic refraction data, we were able to image sub-trappean Mesozoic sediments (〈0.75 km) deposited below the two sequences of thick basaltic flows above the basement. The imaged Mesozoic sediments are expected to contain hydrocarbon because of their wide extension in this sedimentary basin with suitable trapping mechanism due to basalts. The robustness of the velocity image is assessed through numerous tests like velocity perturbations, 2 estimates, rms residuals of traveltime fit, uncertainty estimates through computation of ray-density or hits and series of checkerboard resolution tests with velocity anomalies having different cell size. The thickness of the basalt and the sub-trappean Mesozoic sediments along with the basement geometry obtained from tomography are constrained through ray-trace modelling and pre-stack depth migration (PSDM) of the wide-angle reflection phases for different shot gathers along the profile.

Description: Numerical experiments of convection with grain-damage are used to develop scaling laws for convective heat flow, mantle velocity and plate velocity across the stagnant lid and plate-tectonic regimes. Three main cases are presented in order of increasing complexity: a simple case wherein viscosity is only dependent on grain size, a case where viscosity depends on temperature and grain size, and finally a case where viscosity is temperature and grain size sensitive, and the grain-growth (or healing) is also temperature sensitive. In all cases, convection with grain-damage scales differently than Newtonian convection; whereas the Nusselt number (Nu), typically scales with the reference Rayleigh number, Ra 0 , to the 1/3 power, for grain-damage this exponent is larger because increasing Ra 0 also enhances damage. In addition, Nu, mantle velocity, and plate velocity are also functions of the damage to healing ratio, ( D / H ); increasing D / H increases Nu because more damage leads to more vigorous convection. For the fully realistic case, numerical results show stagnant lid convection, fully mobilized convection that resembles the temperature-independent viscosity case, and partially mobile or transitional convection, depending on D / H , Ra 0 , and the activation energies for viscosity and healing. Applying our scaling laws for the fully realistic case to Earth and Venus we demonstrate that increasing surface temperature dramatically decreases plate speed and heat flow, essentially shutting down plate tectonics, due to increased healing in lithospheric shear zones, as proposed previously. Contrary to many previous studies, the transitional regime between the stagnant lid and fully mobilized regimes is large, and the transition from stagnant lid to mobile convection is gradual and continuous. Thus planets could exhibit a full range of surface mobility, as opposed to the bimodal distribution of fully mobile lid planets and stagnant lid planets that is typically assumed.

Description: Three magnetotelluric (MT) profiles in northwestern Canada cross the central and western segments of Great Slave Lake shear zone (GSLsz), a continental scale strike-slip structure active during the Slave-Rae collision in the Proterozoic. Dimensionality analysis indicates that (i) the resistivity structure is approximately 2-D with a geoelectric strike direction close to the dominant geological strike of N45°E and that (ii) electrical anisotropy may be present in the crust beneath the two southernmost profiles. Isotropic and anisotropic 2-D inversion and isotropic 3-D inversions show different resistivity structures on different segments of the shear zone. The GSLsz is imaged as a high resistivity zone (〉5000  m) that is at least 20 km wide and extends to a depth of at least 50 km on the northern profile. On the southern two profiles, the resistive zone is confined to the upper crust and pierces an east-dipping crustal conductor. Inversions show that this dipping conductor may be anisotropic, likely caused by conductive materials filling a network of fractures with a preferred spatial orientation. These conductive regions would have been disrupted by strike-slip, ductile deformation on the GSLsz that formed granulite to greenschist facies mylonite belts. The pre-dominantly granulite facies mylonites are resistive and explain why the GSLsz appears as a resistive structure piercing the east-dipping anisotropic layer. The absence of a dipping anisotropic/conductive layer on the northern MT profile, located on the central segment of the GSLsz, is consistent with the lack of subduction at this location as predicted by geological and tectonic models.

Description: The 3-D shear velocity structure beneath South India's Dharwar Craton determined from fundamental mode Rayleigh waves phase velocities reveals the existence of anomalously high velocity materials in the depth range of 50–100 km. Tomographic analysis of seismograms recorded on a network of 35 broad-band seismographs shows the uppermost mantle shear wave speeds to be as high as 4.9 km s –1 in the northwestern Dharwar Craton, decreasing both towards the south and the east. Below ~100 km, the shear wave speed beneath the Dharwar Craton is close to the global average shear wave speed at these depths. Limitations of usable Rayleigh phase periods, however, have restricted the analysis to depths of 120 km, precluding the delineation of the lithosphere–asthenosphere boundary in this region. However, pressure–temperature analysis of xenoliths in the region suggests a lithospheric thickness of at least ~185 km during the mid-Proterozoic period. The investigations were motivated by a search for seismic indicators in the shallow mantle beneath the distinctly different parts of the Dharwar Craton otherwise distinguished by their lithologies, ages and crustal structure. Since the ages of cratonic crust and of the associated mantle lithosphere around the globe have been found to be broadly similar and their compositions bimodal in time, any distinguishing features of the various parts of the Dharwar shallow mantle could thus shed light on the craton formation process responsible for stabilizing the craton during the Meso- and Neo-Archean.

Description: Frontier hydrocarbon development projects in the deepwater slopes of the Gulf of Mexico Basin, Santos Basin and Lower Congo Basin all require wells to cross ductile layers of autochthonous or allochthonous salt moving at peak rates of 100 mm yr –1 . The Couette–Poiseuille number is introduced here to help pinpoint the depth of shear stress reversal in such salt layers. For any well-planned through salt, the probable range of creep forces of moving salt needs to be taken into account when designing safety margins and load-factor tolerance of the well casing. Drag forces increase with wellbore diameter, but more significantly with effective viscosity and speed of the creeping salt layer. The potential drag forces on cased wellbores in moving salt sheets are estimated analytically using a range of salt viscosities (10 15 –10 19 Pa s) and creep rates (0–10 mm yr –1 ). Drag on perfectly rigid casing of infinite strength may reach up to 13 Giga Newton per meter wellbore length in salt having a viscosity of 10 19 Pa s. Well designers may delay stress accumulations due to salt drag when flexible casing accommodates some of the early displacement and strain. However, all creeping salt could displace, fracture and disconnect well casing, eventually. The shear strength of typical heavy duty well casing (about 1000 MPa) can be reached due to drag by moving salt. Internal flow of salt will then fracture the casing near salt entry and exit points, but the structural damage is likely to remain unnoticed early in the well-life when the horizontal shift of the wellbore is still negligibly small (at less than 1 cm yr –1 ). Disruption of casing and production flow lines within the anticipated service lifetime of a well remains a significant risk factor within distinct zones of low-viscosity salt which may reach ultrafast creep rates of 100 mm yr –1 .

Description: The Antarctic Peninsula (AP) consists of a long lived and uniquely well preserved magmatic arc system. The broad tectonic structure of the AP arc is well understood. However, magmatic processes occurring along the arc are only constrained by regional geophysical and relatively sparse geological data. Key questions remain about the timing, volume, and structural controls on magma emplacement. We present new high resolution aeromagnetic data across Adelaide Island, on the western margin of the AP revealing the complex structure of the AP arc/forearc boundary. Using digital enhancement, 2-D modelling and 3-D inversion we constrain the form of the magnetic sources at the arc/forearc boundary. Our interpretation of these magnetic data, guided by geological evidence and new zircon U-Pb dating, suggests significant Palaeogene to Neogene magmatism formed ~25 per cent of the upper crust in this region (~7500 km 3 ). Significant structural control on Neogene magma emplacement along the arc/forearc boundary is also revealed. We hypothesize that this Neogene magmatism reflects mantle return flow through a slab window generated by Late Palaeogene cessation of subduction south of Adelaide Island. This mantle process may have affected the final stages of arc magmatism along the AP margin.

Description: The intrusion mechanism and internal structure of sills are still under debate. We present a detailed magnetic study, including anisotropy of magnetic susceptibility and rock magnetic analyses of a Cretaceous (94 Ma), 7-m-thick sill from the Lusitanian Basin in Portugal, the Foz da Fonte sill. The results, from both the top surface and a vertical profile, allow us to propose a model for the magmatic flow pattern and sense of flow. According to their location in the vertical profile, three magnetic fabric domains are identified: (1) at the borders, qualified as chilled margins (~0–50 cm), low anisotropies suggest that low velocity gradients and heterogeneous flow paths occurred during the initial emplacement stages; (2) in the centre of the sill, where low anisotropies are observed, low shear gradients and magma displacement close to pure translation is inferred and (3) in the intermediate zones, high anisotropy values are ascribed to zones having undergone high shear gradients. The mean magnetic lineations from the top surface and basal contact indicate an almost horizontal and NW–SE orientation (azimuth: 310°) which agrees with the preferred orientation of iron oxide grain clusters and with the elongation of vesicles considered as coaxial with the magma flow direction. Moreover, the magnetic foliation planes and the lineations show both a mirror imbrication relative to the average upper and lower border surfaces of the sill, pointing to a flow direction towards the SE. Based on these results and on the interpretation of two seismic reflection lines, we show that the Cabo Raso magnetic anomaly, located 25 km to NW of the FF-sill, is associated to Cretaceous magmatic intrusions from which the sill likely originated. This tectono-magmatic setting is discussed with respect to the West Iberia Late Cretaceous magmatism, integrating magnetic anomalies, isotope chronology and tectonics.

Description: Southern Mendoza and northern Neuquén Provinces, south of the Pampean Shallow Subduction region in western Argentina, are host to the 〈2 Myr Payunia Basaltic Province, which covers ~39 500 km 2 with primarily basaltic intraplate volcanism. This backarc igneous province can be explained by extension due to trench roll-back following steepening of a flat slab that existed in the middle to late Miocene. Magnetotelluric data collected at 37 sites from 67°W to 70°W and 35°S to 38°S are used to probe the source of the Payún Matrú basalts. These data, which require significantly 3-D structure, are inverted with a 3-D non-linear conjugate gradient algorithm that minimizes structure for a given data misfit. We identify two significant electrically conductive structures. One, called the SWAP (shallow western asthenospheric plume) approaches the surface beneath the Payún Matrú Caldera and the Trómen Volcano and dips westward towards the subducted Nazca slab. The second, called the DEEP (deep eastern plume) approaches the surface ~100 km to the southeast of Payún Matrú and dips steeply east to ~400 km depth while remaining above the subducted Nazca slab. We use a variety of model assessment techniques including forward modelling and constrained inversion to test the veracity of these features. We interpret the SWAP as the source of the 〈2 Myr intraplate volcanism. Our model assessment permits but does not require the SWAP to connect to the Nazca slab. The SWAP and DEEP are electrically connected only in the shallow crust, which is likely due to the Neuquén sedimentary basin and not a magmatic process. We propose that the SWAP and DEEP may have been more robustly connected in the past, but that the DEEP was decapitated to form the SWAP when shallow northwestward mantle flow resumed during steepening of the slab. The ~2 Myr basaltic volcanism is the result of this decapitated DEEP magma that had ponded below the crust until extension allowed eruption. The westward dip of the SWAP is interpreted to be the result of shear in the renewed mantle corner flow—this explains why the SWAP and Nazca slab can appear connected, yet there is no recent arc-signature magma in this region.

Description: This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2-D multichannel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a north–south line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100-km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt. The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5–5 km thick at the foot of the margin. They are characterized by seismic velocities from 1.9 to 3.8 km s –1 . Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 and 3.8 km s –1 . The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7–6.1 km s –1 ) and a low gradient lower crust (6.2–7.1 km s –1 ). The upper-mantle velocity is constrained to 7.9 km s –1 . The ocean–continent transition zone is very narrow between 15 and 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper- and lower-crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from three additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin.

Description: For the first time, a deep seismic data set acquired in the frame of the Algerian–French SPIRAL program provides new insights regarding the origin of the westernmost Algerian margin and basin. We performed a tomographic inversion of traveltimes along a 100-km-long wide-angle seismic profile shot over 40 ocean bottom seismometers offshore Mostaganem (Northwestern Algeria). The resulting velocity model and multichannel seismic reflection profiles show a thin (3–4 km thick) oceanic crust. The narrow ocean–continent transition (less than 10 km wide) is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with Miocene to Quaternary sediments. This fault system, as well as the faults organized in a negative-flower structure on the continent side, marks a major strike-slip fault system. The extremely sharp variation of the Moho depth (up to 45 ± 3°) beneath the continental border underscores the absence of continental extension in this area. All these features support the hypothesis that this part of the margin from Oran to Tenes, trending N65–N70°E, is a fossil subduction-transform edge propagator fault, vestige of the propagation of the edge of the Gibraltar subduction zone during the westward migration of the Alborán domain.